Frentiapitce.-] 


THE    COMMON    CRAYFISH 
Astac-us  Jluviattlis,  Male.) 


THE   INTERNATIONAL   SCIENTIFIC   SERIES. 


AN  INTRODUCTION 


TO   THE 


STUDY   OF  ZOOLOGY, 


ILLUSTRATED  BY  THE 


CRAYFISH 


BY 


T.  H.  HUXLEY,  F.  E.  S. 


COLLEGE  ""OF    DENTISTRY 
UNIVERSITY  OF  CALIFORNIA 


WITH    EIGHTY-TWO    ILLUSTRATIONS. 


NEW   YORK   AND    LONDON 

D.  APPLETON   AND   COMPANY 

1920 


'  Aid  8ei  /KTJ  iv<T\epaiveiv  iraifiiKWS  rr\v  trepi  rfiv  aTiftOT/pwv  (Jwwv  InttTitetyu''   Jv  iraun 
ydp  TOIS  <J»v<riKots  weari  TI  flow/Maorif."— ARISTOTLE,  Dt  Partibus,  I.  5. 


•'  Qui  enim  Autorum  verba  legentes,  rerum  ipsarum  imagines  (eorum  verbis  com- 
prehensa)  seiisibus  propriis  non  abstrahunt,  hi  non  veras  Ideas,  sed  falsa  Idola  et 
phantasmata  inania  mente  concipiunt 

"  Insusurro  itaque  in  aurem  tibi  (amice  Lector !)  ut  qusecunque  a  nobis  in  hisce  .... 
excrcitationibus  tractabuntur,  ad  exactam  experientise  trutinam  pensites :  fidemque 
iis  non  aliter  adhibeas,  nisi  quatenus  eadem  indubitato  sensunm  testimonio  firmissime 
stab  liri  deprehenderis."— HABVEY.  Exercitationes  de  Generation*  Prcefatio. 


"  La  seule  et  vraie  Science  est  la  connaissance  des  faits :  1'esprit  ne  peut  pas  y 
suppleer  et  les  faits  sontdansles  sciences  ce  qu'est  1' experience  dans  la  vie  civile." 

"  Le  seul'et  le  vrai  moyen  d'avancer  la  science  est  de  travailler  a  la  description  et 
a  1'histoire  des  differentes  choses  qui  en  font  1'objet."  —  BUFFON.  Discours  de  la 
maniere  d'etudier  et  de  trailer  I'Histoire  Naturelle. 


"  Ebenso  hat  mich  auch  die  genauere  Untersuchung  unsers  Krebses  gelehret,  dass, 
so  gemein  und  geringschatzig  solcher  auch  den  meisten  zu  seyn  scheinet,  sich  an 
selbigera  doch  so  viel  Wunderbares  findet,  dass  es  auch  den  grossten  Naturforscher 
schwer  fallen  sollte  solc-hes  alles  deutlich  zu  beschreiben."— ROESEL  v.  ROSENHOF. 
Insecten  Belustigung&n. — "Der  Flusskrebt  hiesiges  Landet  rn.it  seinen  merkwurdigen 
Eigenschaften." 


PEEFACE. 

IN  writing  this  book  about  Crayfishes  it  has  not 
been  my  intention  to  compose  a  zoological  mono- 
graph on  that  group  of  animals.  Such  a  work,  to 
be  worthy  of  the  name,  would  require  the  devotion 
of  years  of  patient  study  to  a  mass  of  materials 
collected  from  many  parts  of  the  world.  Nor  has 
it  been  my  ambition  to  write  a  treatise  upon 
our  English  crayfish,  which  should  in  any  way  pro- 
voke comparison  with  the  memorable  labours  of 
Lyonet,  Bojanus,  or  Strauss  Durckheim,  upon  the 
willow  caterpillar,  the  tortoise,  and  the  cockchafer. 
What  I  have  had  in  view  is  a  much  humbler,  though 
perhaps,  in  the  present  state  of  science,  not  less  use- 
ful object.  I  have  desired,  in  fact,  to  show  how 
the  careful  study  of  one  of  the  commonest  and  most 
insignificant  of  animals,  leads  us,  step  by  step,  from 
every-day  knowledge  to  the  widest  generalizations 


Yl  PREFACE. 

and  the  most  difficult  problems  of  zoology;    and, 
indeed,  of  biological  science  in  general. 

It  is  for  this  reason  that  I  have  termed  the  book 
an  "  Introduction  to  Zoology."  For,  whoever  will 
follow  its  pages,  crayfish  in  hand,  and  will  try  to 
verify  for  himself  the  statements  which  it  contains, 
will  find  himself  brought  face  to  face  with  all  the 
great  zoological  questions  which  excite  so  lively  an 
interest  at  the  present  day ;  he  will  understand  the 
method  by  which  alone  we  can  hope  to  attain  to 
satisfactory  answers  of  these  questions;  and,  finally, 
he  will  appreciate  the  justice  of  Diderot's  remark, 
"  II  faut  £tre  profond  dans  Part  ou  dans  la  science 
pour  en  bien  posseder  les  elements." 

And  these  benefits  will  accrue  to  the  student 
whatever  shortcomings  and  errors  in  the  work  itself 
may  be  made  apparent  by  the  process  of  verification. 
"  Common  and  lowly  as  most  may  think  the  cray- 
fish," well  says  Eoesel  von  Eosenhof,  "it  is  yet  so 
full  of  wonders  that  the  greatest  naturalist  may  be 
puzzled  to  give  a  clear  account  of  it."  But  only 


PREFACE.  Vi 

the  broad  facts  of  the  case  are  of  fundamental  im- 
portance; and,  so  far  as  these  are  concerned,  I  ven- 
ture to  hope  that  no  error  has  slipped  into  my 
statement  of  them.  As  for  the  details,  it  must  be 
remembered,  not  only  that  some  omission  or  mis- 
take is  almost  unavoidable,  but  that  new  lights 
come  with  new  methods  of  investigation ;  and  that 
better  modes  of  statement  follow  upon  the  improve- 
ment of  our  general  views  introduced  by  the  gradual 
widening  of  our  knowledge. 

I  sincerely  hope  that  such  amplifications  and 
rectifications  may  speedily  abound;  and  that  this 
sketch  may  be  the  means  of  directing  the  attention  of 
observers  in  all  parts  of  the  world  to  the  crayfishes. 
Combined  efforts  will  soon  furnish  the  answers  to 
many  questions  which  a  single  worker  can  merely 
state;  and,  by  completing  the  history  of  one  group 
of  animals,  secure  the  foundation  of  the  whole 
of  biological  science. 

In  the  Appendix,  I  have  added  a  few  rotes  re- 
specting points  of  detail  with  which  I  thought  it 


Vlll  PREFACE 

unnecessary  to  burden  the  text ;  and,  under  the 
head  of  Bibliography,  I  have  given  some  references 
to  the  literature  of  the  subject  which  may  be  useful 
to  those  who  wish  to  follow  it  out  more  fully. 

I  am  indebted  to  Mr.  T.  J.  Parker,  demonstrator 
of  my  biological  class,  for  several  anatomical  draw- 
ings; and  for  valuable  aid  in  supervising  the 
execution  of  the  woodcuts,  and  in  seeing  the  work 
through  the  press. 

Mr.  Cooper  has  had  charge  of  the  illustrations, 
and  I  am  indebted  to  him  and  to  Mr.  Coombs, 
the  accurate  and  skilful  draughtsman  to  whom 
the  more  difficult  subjects  were  entrusted,  for 
such  excellent  specimens  of  xylographic  art  as 
the  figures  of  the  Crab,  Lobster,  Eock  Lobster, 
and  Norway  Lobster. 

T.  H.  H. 


LONDON, 

November,  1873. 


CONTENTS. 


PBEFACE r 

LIST  OF  WOODCUTS ri 

CHAPTER   I. 
THE  NATURAL  HISTORY  OF  THE  COMMON  CRAYFISH    .       .       •        1 

CHAPTER   II. 

THE  PHYSIOLOGY  OF  THE  COMMON  CRAYFISH.     THE  MECHANISM 

BY  WHICH  THE  PARTS  OF  THE  LIVING  ENGINE  ARE  SUPPLIED 
WITH  THE  MATERIALS  NECESSARY  FOR  THEIR  MAINTENANCE 
AND  GROWTH 46 

CHAPTER  III. 

THE  PHYSIOLOGY  OF  THE  COMMON  CRAYFISH.  THE  MECHANISM 
BY  WHICH  THE  LIVING  ORGANISM  ADJUSTS  ITSELF  TO  SUB- 
BOUNDING  CONDITIONS  AND  REPRODUCES  ITSELF  ...  87 

CHAPTER  IV. 

THB  MORPHOLOGY  OF  THE  COMMON  CRAYFISH.  T^B  STRUCTURE 
AND  THE  DEVELOPMENT  OF  THB  INDIVIDUAL  .  .  137 


CONTENTS. 


CHAPTER  V. 

MM 

THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH.    THE  STRUC- 
TURE   AND    THE    DEVELOPMENT    OF  THE  CRAYFISH  COMPARED 

THOSE  OF   OTHER  LIVING   BEINGS    .  ,      227 


CHAPTER   VI. 

THE  DISTRIBUTION  AND  THE  ^ETIOLOGY  OF  THE  CRAYFISHES  .     ,    288 


NOTES 34? 

BIBLIOGRAPHY     .        . .     857 

INDEX .  *«8 


LIST    OF    WOODCUTS. 


PAOB 

Frontispiece.      THE  COMMON  CRAYFISH,  Astacus  fluviatilis,  (MALE) 


'IG. 

1.   Astacus  fluviatilis. 

SIDE    VIEW   OF   THE    MALE. 

6 

H 

2. 

DOUSAL  VIEWS  OF  MALE  AND  FEMALE 

18 

,, 

3.        ,, 

VENTRAL  VIEWS  OF  MALE  AND  FEMALE 

21 

„ 

*•       »»              »» 

THE  GILLS    .        .        .        ... 

26 

» 

5.       „ 

DISSECTION  FROM  THE  DORSAL  SIDE 

(MALE)        .        .        ... 

28 

„ 

«.       „ 

LONGITUDINAL  VERTICAL  SECTION  OF 

THE   ALIMENTARY    CANAL           .       . 

29 

>» 

7.       „ 

A   GASTROLITH   OR    "  CRAB'S  EYE"      . 

80 

»> 

8.       „ 

ATTACHMENT   OF    YOUNG  TO  SWIM- 

MERET   OF   MOTHER             .            .      . 

41 

»» 

9-       „ 

STRUCTURE  OF  THE  STOMACH   .        . 

53 

„ 

10.     „ 

LONGITUDINAL  SECTION  OF  THE  STO- 

,  « 

56 

»> 

11-       .» 

EOOF  OF  THE  STOMACH,  FROM  WITHIN 

60 

M 

12.       „ 

DISSECTION  FROM  THE  SIDE  (MALE). 

62 

I) 

13.       „ 

ALIMENTARY  CANAL  FROM  ABOVE     . 

65 

M 

14.       „ 

BLOOD  CORPUSCLES    .... 

68 

„ 

15-        »              »> 

TRANSVERSE  SECTION  OF  THORAX    .. 

70 

M 

16.        „ 

THE  HEART 

72 

„ 

17-       „ 

STRUCTURE  OF  THE  GILLS            .    . 

76 

„ 

18.       H 

THE   GREEN   GLAND    .... 

83 

xii  LIST  OF  WOODCUTS. 


7iG.  19.  Astacus  fluviatilii 

?.    MUSCULAR  TISSUE        .       .       .    . 

91 

,, 

20. 

H 

ff 

MUSCLES  OF  CHELA  .... 

93 

99 

21. 

II 

II 

ARTICULATION    OF    ABDOMINAL    SO- 

97 

|| 

22. 

•1 

M 

MUSCULAR  SYSTEM              .         .         . 

100 

II 

23. 

II 

II 

NERVE  FIBRES     

102 

N 

24. 

fl 

ff 

NERVE  GANGLIA       .       •       .        . 

103 

ii 

25. 

ff 

If 

NERVOUS  SYSTEM         .                .    . 

104 

„ 

26. 

ff 

ff 

OLFACTORY  AND  AUDITORY   ORGANS 

114 

07 

117 

if 

At  • 

28. 

fl 
ff 

ff 
ff 

STRUCTURE  OF  EYE      .        .       •    • 

119 

M 

29. 

„ 

ff 

DIAGRAM  OF  EYE     .... 

123 

N 

30. 

ff 

ff 

FEMALE  REPRODUCTIVE  ORGANS  .    . 

129 

f> 

31. 

ff 

ff 

MALE  REPRODUCTIVE  ORGANS  , 

130 

,, 

32. 

ff 

ff 

STRUCTURE  OF  OVARY  .        ... 

131 

H 

33. 

ff    • 

ff 

STRUCTURE  OF  TESTIS      .        . 

132 

„ 

34. 

ff 

ff 

SPERMATOZOA       

134 

ff 

35. 

if 

If 

THE  LAST  THORACIC  STERNUM   IN  THE 

MALE  AND   FEMALE       . 

136 

w 

36. 

ff 

ff 

TRANSVERSE   SECTION   OF   ABDOMEN 

142 

N 

37. 

fl 

ff 

ABDOMINAL  APPENDAGES      .        .     . 

144 

w 

38. 

ff 

fl 

CONNECTION  BETWEEN  THORAX  AND 

ABDOMEN      

151 

ff 

39. 

II 

II 

CEPHALOTHORACIC  STERNA  AND  EN- 

DOPHRAGMAL  SYSTEM      \           .      . 

153 

ff 

40. 

II 

II 

OPHTHALMIC  AND  ANTENNULARY  so- 

156 

„ 

41. 

ff 

ff 

THE  ROSTRUM      

157 

ft 

42. 

II 

II 

A  SEGMENT  OF  THE  ENDOPHRAGMAL 

SYSTEM           

159 

fc 

43. 

II 

II 

LONGITUDINAL  SECTION  OF  CEPHALO- 

162 

II 

44. 

II 

M 

THE  THIRD   MAXILLIPEDK. 

164 

LIST  OF   WOODCUTS. 


xni 


FIG.  45.  Astacus  fluviatilis.     THE    FIRST   AND    SECOND    MAXILLI- 

PEDES       .  .  .  .  .       . 

THE   SECOND   AMBULATORY  LEG  . 
THE   MANDIBLE  AND   MAXILUB       .      . 
THE     EYE-STALK,      ANTENNULE,     AND 

ANTENNA      

BLOOD  CORPUSCLES 


46. 
47. 

48. 


49. 
50. 
51. 
52. 
53. 
54. 
55. 
56. 
57. 
58. 
59. 
60. 

61. 


62. 

63. 
64. 
65. 
66. 

67. 
68. 


PACK 

166 
169 
171 

172 

176 


EPITHELIUM 178 

CONNECTIVE  TISSUE  .  .  .  .  179 
MUSCULAR  TISSUE  .  .  .  .181 
MUSCULAR  TISSUE  .  ...  182 
NERVF  GANGLIA  .  .  .  .188 

NERVE  FIBRES 189 

CUTICULAR  TISSUE  .  .  .  .191 
SECTIONS  OF  EMBRYOS  .  ...  208 
EARLIER  STAGES  OF  DEVELOPMENT  210 
LATER  STAGES  OF  DEVELOPMENT  .  216 
NEWLY  HATCHED  YOUNG  .  .  .  220 

(torrentium.  \  COMPARATIVE  VIEWS  OF  THE  CARA- 
nobilis.  L  PACE,  THIRD  ABDOMINAL  SOMITE, 
nigrescens.  }  AND  TELSON  .  .  .  .233 
ftorrentium.  \  COMPARATIVE  VIEWS  OF  THE  FIRST 
nobilis.  [•  AND  SECOND  ABDOMINAL  APPEN- 
nigrescens.  }  DAGES  OF  THE  MALE  .  .  .  245 

Cambarus  Clarkii 248 

Parastacus  brasiliensis .        .        .        .        .        .        ..250 

Astacoides  madagascarensis 251 

DIAGRAM   OF   THE  MORPHOLOGICAL  RELATIONS  OF  THE 

AstacidoR 253 

Hom.arus  vulgaris     ........     258 

I  Parastacus.  \ 

\Nephrops.     t  PoDOBRANCHLS 259 

1  Palcemon.    J 


ri?  LIST  OF  WOODCUTS. 

PAGB 

FIG.  69.  Nephrops  norvegicus .        ••••••.  260 

„     70.  Palinurus  vulgaris        •••••••.  262 

„     71.  Palcemon  jamaicensia        ••«....  269 

„     72.  Cancer  pagurus 273 

„    73.  Penceus 281 

„    74.  Cancer  pagvtrus.    DEVELOPMENT  ......  282 

„     75.  Astacus  Uptodactylis 301 

„     76.  Australian  Crayfish 307 

„      77.    MAP  OP  THE  DISTRIBUTION  OF  CRAYFISHES    .           .          .  309 

„    78.  Cambana.    WALKING  LEO.         .        .        .        .        ,    .  312 

„    79.  Palcemon  jamaicensis        .        ,        .        •        .        •        .  329 

j  Pseudastacus  pustulosus  J 

I  Eryma  modestiformis      i 

M    II.  Hoplaparia  longimana      .                 842 


THE  CRAYFISH: 

AN  INTRODUCTION  TO  THE  STUDY  OF  ZOOLOGY. 


CHAPTER  I. 

THE   NATURAL   HISTORY   OF   THE    COMMON   CRAYFISH 
(Astacus  fluviatilis.) 

MANY  persons  seem  to  believe  that  what  is  teamed 
Science  is  of  a  widely  different  nature  from  ordinary 
knowledge,  and  that  the  methods  by  which  scientific 
truths  are  ascertained  involve  mental  operations  of  a 
recondite  and  mysterious  nature,  comprehensible  only  by 
the  initiated,  and  as  distinct  in  their  character  as  in 
their  subject  matter,  from  the  processes  by  which  we 
discriminate  between  fact  and  fancy  in  ordinary  life. 

But  any  one  who  looks  into  the  matter  attentively  will 
soon  perceive  that  there  is  no  solid  foundation  for  the 
belief  that  the  realm  of  science  is  thus  shut  off  fi  om  that 
of  common  sense ;  or  that  the  mode  of  investigation  which 
yields  such  wonderful  results  to  the  scientific  inves- 
tigator, is  different  in  kind  from  that  which  is  employed 


2  '     THE  NATURAL  ^HfStORY   OF  THE  COMMON   CRAYFISH 

for  the  commonest  purposes  of  everyday  existence. 
Common  sense  is  science  exactly  in  so  far  as  it  fulfils 
the  ideal  of  common  sense ;  that  is,  sees  facts  as  they 
are,  or,  at  any  rate,  without  the  distortion  of  prejudice, 
and  reasons  from  them  in  accordance  with  the  dictates 
of  sound  judgment.  And  science  is  simply  common  sense 
at  its  hest ;  that  is,  rigidly  accurate  in  observation,  and 
merciless  to  fallacy  in  logic. 

Whoso  will  question  the  validity  of  the  conclusions  of 
sound  science,  must  be  prepared  to  carry  his  scepticism 
a  long  way ;  for  it  may  be  safely  affirmed,  that  there  is 
hardly  any  of  those  decisions  of  common  sense  on 
which  men  stake  their  all  in  practical  life,  which  can 
justify  itself  so  thoroughly  on  common  sense  principles, 
as  the  broad  truths  of  science  can  be  justified. 

The  conclusion  drawn  from  due  consideration  of  the 
nature  of  the  case  is  verified  by  historical  inquiry  ;  and 
the  historian  of  every  science  traces  back  its  roots  to  the 
primary  stock  of  common  information  possessed  by  all 
mankind. 

In  its  earliest  development  knowledge  is  self-sown. 
Impressions  force  themselves  upon  men's  senses  whether 
they  will  or  not,  and  often  against  their  will.  The 
amount  of  interest  which  these  impressions  awaken  is 
determined  by  the  coarser  pains  and  pleasures  which 
they  carry  in  their  train,  or  by  mere  curiosity;  and 
reason  deals  with  the  materials  supplied  to  it  as  far  aa 
that  interest  carries  it,  and  no  farther.  Such  common 


COMMON   KNOWLEDGE  AND  SCIENCE.  3 

knowledge  is  rather  brought  than  sought ;  and  such 
ratiocination  is  little  more  than  the  working  of  a  blind 
intellectual  instinct. 

It  is  only  when  the  mind  passes  beyond  this  condition 
that  it  begins  to  evolve  science.  When  simple  curiosity 
passes  into  the  love  of  knowledge  as  such,  and  the 
gratification  of  the  aesthetic  sense  of  the  beauty  of  com- 
pleteness and  accuracy  seems  more  desirable  than  the 
easy  indolence  of  ignorance ;  when  the  finding  out  of 
the  causes  of  things  becomes  a  source  of  joy,  and  he 
is  counted  happy  who  is  successful  in  the  search ;  common 
knowledge  of  nature  passes  into  what  our  forefathers 
called  Natural  History,  from  whence  there  is  but  a  step 
to  that  which  used  to  be  termed  Natural  Philosophy,  and 
now  passes  by  the  name  of  Physical  Science. 

In  this  final  stage  of  knowledge,  the  phenomena  of 
nature  are  regarded  as  one  continuous  series  of  causes 
and  effects ;  and  the  ultimate  object  of  science  is  to  trace 
out  that  series,  from  the  term  which  is  nearest  to  us,  to 
that  which  is  at  the  furthest  limit  accessible  to  our  means 
of  investigation. 

The  course  of  nature  as  it  is,  as  it  has  been,  and  as  it 
Till  be,  is  the  object  of  scientific  inquiry ;  whatever  lies 
beyond,  above,  or  below  this,  is  outside  science.  But 
the  philosopher  need  not  despair  at  the  limitation  of  his 
field  of  labour  :  in  relation  to  the  human  mind  Nature  is 
boundless ;  and,  though  nowhere  inaccessible,  she  is 
everywhere  unfathomable. 


4          THE  NATURAL   HISTORY   OF  THE   COMMON   CRAYFISH, 

The  Biological  Sciences  embody  the  great  multitude 
of  truths  which  have  been  ascertained  respecting  living 
beings ;  and  as  there  are  two  chief  kinds  of  living  things, 
animals  and  plants,  so  Biology  is,  for  convenience  sak  e, 
divided  into  two  main  branches,  Zoology  and  Botany. 

Each  of  these  branches  of  Biology  has  passed  through 
the  three  stages  of  development,  which  are  common  to 
all  the  sciences  ;  and,  at  the  present  time,  each  is  in  these 
different  stages  in  different  minds.  Every  country  boy 
possesses  more  or  less  information  respecting  the  plants 
and  animals  which  coine  under  his  notice,  in  the  stage 
of  common  knowledge;  a  good  many  persons  have 
acquired  more  or  less  of  that  accurate,  but  necessarily 
incomplete  and  unmethodised  knowledge,  which  is  under- 
stood by  Natural  History ;  while  a  few  have  reached  the 
purely  scientific  stage,  and,  as  Zoologists  and  Botanists, 
strive  towards  the  perfection  of  Biology  as  a  branch  of 
Ph}rsical  Science. 

Historically,  common  knowledge  is  represented  by  the 
allusions  to  animals  and  plants  in  ancient  literature  ; 
while  Natural  History,  more  or  less  grading  into  Biology, 
meets  us  in  the  works  of  Aristotle,  and  his  continuators 
in  the  Middle  Ages,  Rondoletius,  Aldrovandus,  and  their 
contemporaries  and  successors.  But  the  conscious  at- 
tempt to  construct  a  complete  science  of  Biology  hardly 
dates  further  back  than  Treviranus  and  Lamarck,  at 
the  beginning  of  this  century,  while  it  has  received  its 
strongest  impulse,  in  our  own  day,  from  Darwin. 


COMMON   KNOWLEDGE  OF  THE   CRAYFISH.  5 

My  purpose,  in  the  present  work,  is  to  exemplify  the 
general  truths  respecting  the  development  of  zoological 
science  which  have  just  heen  stated  by  the  study  of  a 
special  case ;  and,  to  this  end,  I  have  selected  an  animal, 
the  Common  Crayfish,  which,  taking  it  altogether,  is 
better  fitted  for  my  purpose  than  any  other. 

It  is  readily  obtained,*  and  all  the  most  important 
points  of  its  construction  are  easily  deciphered ;  hence, 
those  who  read  what  follows  will  have  no  difficulty  in 
ascertaining  whether  the  statements  correspond  with  facts 
or  not.  And  unless  my  readers  are  prepared  to  take  this 
much  trouble,  they  may  almost  as  well  shut  the  book ; 
for  nothing  is  truer  than  Harvey's  dictum,  that  those 
who  read  without  acquiring  distinct  images  of  the  things 
about  which  they  read,  by  the  help  of  their  own  senses, 
gather  no  real  knowledge,  but  conceive  mere  phantoms 
and  idola. 

It  is  a  matter  of  common  information  that  a  number  of 
our  streams  and  rivulets  harbour  small  animals,  rarely 
more  than  three  or  four  inches  long,  which  are  very  similar 
to  little  lobsters,  except  that  they  are  usually  of  a  dull, 
greenish  or  brownish  colour,  generally  diversified  with 
pale  yellow  on  the  under  side  of  the  body,  and  some- 
times with  red  on  the  limbs.  In  rare  cases,  their 

*  If  crayfish  are  not  to  be  had,  a  lobster  will  be  found  to  answer  to 
the  description  of  the  former,  in  almost  all  points  ;  but  the  gills  and 
the  abdominal  appendages  present  differences  ;  and  the  last  thoracic 
somite  is  united  with  the  rest  in  the  lobster.  (See  Chap.  V.) 


G         THE  NATURAL   HISTORY   OF  THE   COMMON   CRAYFISH. 

general  hue  may  be  red  or  blue.  These  are  "  cray- 
fishes," and  they  oannot  possibly  be  mistaken  for  any 
other  inhabitants  of  our  fresh  waters. 


XV 


FIG.  1. — Astacus  fluviatilis, — Side  view  of  a  male  specimen  (nat.  size) : — 
bff,  branchiostegite  ;  eg,  cervical  groove  ;  r,  rostrum  ;  f,  telson. — 
1,  eye-stalk  ;  2,  antennule ;  3,  antenna  ;  9,  external  maxillipede  ; 
10,  forceps;  14,  last  ambulatory  leg;  17,  third  abdominal  ap- 
pendage ;  20,  lateral  lobe  of  the  tail-fin,  or  sixth  abdominal 
appendage  ;  xv,  the  first  ;  and  xx,  the  last  abdominal  somite. 
In  this  and  in  succeeding  figures  the  numbers  of  the  somites  are 
given  in  Roman,  those  of  the  appendages  in  ordinary  numerals. 


The  animals  may  be  seen  walking  along  the  bottom 
of  the  shallow  waters  which  they  prefer,  by  means  of  four 
pairs  of  jointed  legs  (fig.  1);  but,  if  alarmed,  they  swim 


MALE  AND   FEMALE   CRAYFISHES.  7 

backwards  with  rapid  jerks,  propelled  by  the  strokes  of  a 
broad,  fan-shaped  flipper,  which  terminates  the  hinder 
end  of  the  body  (fig.  1,  t,  20).  In  front  of  the  four  pairs 
of  legs,  which  are  used  in  walking,  there  is  a  pair  of 
limbs  of  a  much  more  massive  character,  each  of  which 
ends  in  two  claws  disposed  in  such  a  manner  as  to 
constitute  a  powerful  pincer  (fig.  1;  10).  These 
claws  are  the  chief  weapons  of  offence  and  defence 
of  the  crayfish,  and  those  who  handle  them  incautiously 
will  discover  that  their  grip  is  by  no  means  to  be  des- 
pised, and  indicates  a  good  deal  of  disposable  energy. 
A  sort  of  shield  covers  the  front  part  of  the  body, 
and  ends  in  a  sharp  projecting  spine  in  the  middle 
line  (r).  On  each  side  of  this  is  an  eye,  mounted  on  a 
movable  stalk  (1),  which  can  be  turned  in  any  direction : 
behind  the  eyes  follow  two  pairs  of  feelers  ;  in  one  of 
these,  the  feeler  ends  in  two,  short,  jointed  filaments  (2)  ; 
\vhile,  in  the  other,  it  terminates  in  a  single,  many-jointed 
filament,  like  a  whip-lash,  which  is  more  than  half  the 
length  of  the  body  (3).  Sometimes  turned  backwards, 
sometimes  sweeping  forwards,  these  long  feelers  con- 
tinually explore  a  considerable  area  around  the  body  of 
the  crayfish. 

If  a  number  of  crayfishes,  of  about  the  same  size,  are 
compared  together,  it  will  easily  be  seen  that  they  fall 
into  two  sets  ;  the  jointed  tail  being  much  broader, 
especially  in  the  middle,  in  the  one  set  than  in  the 
other  (fig.  2).  The  broad-tailed  crayfishes  are  the 


8         THE   NATURAL   HISTORY   OF  THE  COMMON    CRAYFISH. 

females,  the  others  the  males.  And  the  latter  may 
be  still  more  easily  known  by  the  possession  of  four 
curved  styles,  attached  to  the  under  face  of  the  first 
two  rings  of  the  tail,  which  are  turned  forwards  between 
the  hinder  legs,  on  the  under  side  of  the  body  (fig.  3,  A; 
16,  16).  In  the  female,  there  are  mere  soft  filaments  in 
the  place  of  the  first  pair  of  styles  (fig.  3,  B  ;  25). 

Crayfishes  do  not  inhabit  every  British  river,  and  even 
where  they  are  known  to  abound,  it  is  not  easy  to  find 
them  at  all  times  of  the  year.  In  granite  districts  and 
others,  in  which  the  soil  jields  little  or  no  calcareous 
matter  to  the  waters  which  flow  over  it,  crayfishes  do 
not  occur.  They  are  intolerant  of  great  heat  and  of 
much  sunshine  ;  they  are  therefore  most  active  towards 
the  evening,  while  they  shelter  themselves  under  the 
shade  of  stones  and  banks  during  the  day.  It  has  been 
observed  that  they  frequent  those  pails  of  a  river  which 
run  north  and  south,  less  than  those  which  have  an 
easterly  and  westerly  direction,  inasmuch  as  the  latter 
yield  more  shade  from  the  mid-day  sun. 

During  the  depth  of  winter,  crayfishes  are  rarely  to 
be  seen  about  in  a  stream ;  but  they  may  be  found 
in  abundance  in  its  banks,  in  natural  crevices  and  in 
burrows  which  they  dig  for  themselves.  The  burrows 
may  be  from  a  few  inches  to  more  than  a  yard  deep, 
and  it  has  been  noticed  that,  if  the  waters  are  liable 
to  freeze,  the  burrows  are  deeper  and  further  from 
the  surface  than  otherwise.  Where  the  soil,  through 


THE   FOOD   OF  THE  CRAYFISH.  9 

which  a  stream  haunted  by  crayfishes  runs,  is  soft 
and  peaty,  the  crayfishes  work  their  way  into  it  in  all 
directions,  and  thousands  of  them,  of  all  sizes,  may  be 
dug  out,  even  at  a  considerable  distance  from  the  banks. 

It  does  not  appear  that  crayfishes  fall  into  a  state  of 
torpor  in  the  winter,  and  thus  "hybernate"  in  the  strict 
sense  of  the  word.  At  any  rate,  so  long  as  the  weather 
is  open,  the  crayfish  lies  at  the  mouth  of  his  burrow, 
barring  the  entrance  with  his  great  claws,  and  with  pro- 
truded feelers  keeps  careful  watch  on  the  passers-by. 
Larvae  of  insects,  water- snails,  tadpoles,  or  frogs,  which 
come  within  reach,  are  suddenly  seized  and  devoured, 
and  it  is  averred  that  the  water-rat  is  liable  to  the  same 
fate.  Passing  too  near  the  fatal  den,  possibly  in  search 
of  a  stray  crayfish,  whose  flavour  he  highly  appreciates, 
the  vole  is  himself  seized  and  held  till  he  is  suffocated, 
when  his  captor  easily  reverses  the  conditions  of  the  anti- 
cipated meal. 

In  fact,  few  things  in  the  way  of  food  are  amiss  to 
the  crayfish ;  living  or  dead,  fresh  or  carrion,  animal  or 
vegetable,  it  is  all  one.  Calcareous  plants,  such  as  the 
stoneworts  (Char a),  are  highly  acceptable;  so  are  any  kinds 
of  succulent  roots,  such  as  carrots;  and  it  is  said  that 
crayfish  sometimes  make  short  excursions  inland,  in 
search  of  vegetable  food.  Snails  are  devoured,  shells 
and  all ;  the  cast  coats  of  other  crayfish  are  turned  to 
account  as  supplies  of  needful  calcareous  matter ;  and 
the  unprotected  or  weakly  member  of  the  family  is 


10       THE   NATURAL  HISTORY  OF  THE   COMMON  CRAYFISH, 

not  spared.  Crayfishes,  in  fact,  are  guilty  of  cauni- 
ball  sin  in  its  worst  form  ;  and  a  French  observer  pa- 
thetically remarks,  that,  under  certain  circumstances, 
the  males  " meconnaissent  les plus  saints  devoirs;"  and, 
not  content  with  mutilating  or  killing  their  spouses, 
after  the  fashion  of  animals  of  higher  moral  pretensions, 
they  descend  to  the  lowest  depths  of  utilitarian  turpitude, 
and  finish  by  eating  them. 

In  the  depth  of  winter,  however,  the  most  alert  of 
crayfish  can  find  little  enough  food ;  and  hence,  when 
they  emerge  from  their  hiding-places  in  the  first  warm 
days  of  spring,  usually  about  March,  the  crayfishes  are  in 
poor  condition. 

At  this  time,  the  females  are  found  to  be  laden  with 
eggs,  of  which  from  one  to  two  hundred  are  attached  be- 
neath the  tail,  and  look  like  a  mass  of  minute  berries 
(fig.  3,  B).  In  May  or  June,  these  eggs  are  hatched,  and 
give  rise  to  minute  young,  which  are  sometimes  to  be 
found  attached  beneath  the  tail  of  the  mother,  under 
whose  protection  they  spend  the  first  few  days  of  their 
existence. 

In  this  country,  we  do  not  set  much  store  upon  cray- 
fishes as  an  article  of  food,  but  on  the  Continent,  and 
especially  in  France,  they  are  in  great  request.  Paris 
alone,  with  its  two  millions  of  inhabitants,  consumes 
annually  from  five  to  six  millions  of  crayfishes,  and  pays 
about  .£16,000  for  them.  The  natural  productivity  of  the 
rivers  of  France  has  long  been  inadequate  to  supply  the 


THE  ORIGIN   OF  THE  WORD   CRAYFISH.  11 

demand  for  these  delicacies  ;  and  hence,  not  only  are  large 
quantities  imported  from  Germany,  and  elsewhere,  but 
the  artificial  cultivation  of  crayfish  has  been  successfully 
attempted  on  a  considerable  scale. 

Crayfishes  are  caught  in  various  ways ;  sometimes  the 
fisherman  simply  wades  in  the  water  and  drags  them  out 
of  their  burrows ;  more  commonly,  hoop-nets  baited  with 
frogs  are  let  down  into  the  water  and  rapidly  drawn  up, 
when  there  is  reason  to  think  that  crayfish  have  been 
attracted  to  the  bait ;  or  fires  are  lighted  on  the  banks  at 
night,  and  the  crayfish,  which  are  attracted,  like  moths, 
to  the  unwonted  illumination,  are  scooped  out  with  the 
hand  or  with  nets. 

Thus  far,  our  information  respecting  the  crayfish  is 
such  as  would  be  forced  upon  anyone  who  dealt  in  cray- 
fishes, or  lived  in  a  district  in  which  they  were  commonly 
used  for  food.  It  is  common  knowledge.  Let  us  now 
try  to  push  our  acquaintance  with  what  is  to  be  learned 
about  the  animal  a  little  further,  so  as  to  be  able  to  give 
an  account  of  its  Natural  History,  such  as  might  have 
been  furnished  by  Buffon  if  he  had  dealt  with  the  subject. 

There  is  an  inquiry  which  does  not  strictly  lie 
within  the  province  of  physical  science,  and  yet  suggests 
itself  naturally  enough  at  the  outset  of  a  natural  history. 

The  animal  we  are  considering  has  two  names, 
one  common,  Craiifah,  the  other  technical,  Astacus  flu- 
viatilis.  How  has  it  come  by  these  two  names,  and  why, 


12       THE  NATURAL   HISTORY  OF  THE  COMMON   CRAYFISH, 

having  a  common  English  name  for  it  already,  should 
naturalists  call  it  by  another  appellation  derived  from  a 
foreign  tongue  ? 

The  origin  of  the  common  name,  "crayfish,"  involves 
some  curious  questions  of  etymology,  and  indeed,  of  his- 
tory. It  might  readily  be  supposed  that  the  word  "cray" 
had  a  meaning  of  its  own,  and  qualified  the  substantive 
"fish  "-as  "jelly"  and  "cod"  in  "jellyfish"  and  "codfish." 
But  this  certainly  is  not  the  case.  The  old  English 
method  of  writing  the  word  was  "  crevis  "  or  "  crevice," 
and  the  "  cray  "  is  simply  a  phonetic  spelling  of  the  syl- 
lable "  ere,"  in  which  the  "  e  "  was  formerly  pronounced 
as  all  the  world,  except  ourselves,  now  pronounce  that 
vowel.  While  "  fish  "  is  the  "  vis  "  insensibly  modified 
to  suit  our  knowledge  of  the  thing  as  an  aquatic 
animal. 

Now  "  crevis  "  is  clearly  one  of  two  things.  Either  it 
is  a  modification  of  the  French  name  "  ecrevisse,"  or  of 
the  Low  Dutch  name  "  crevik,"  by  which  the  crayfish  is 
known  in  these  languages.  The  former  derivation  is  that 
usually  given,  and,  if  it  be  correct,  we  must  refer  "cray- 
fish" to  the  same  category  as  "mutton,"  "beef,"  and 
"pork/'  all  of  which  are  French  equivalents,  introduced 
by  the  Normans,  for  the  "sheep's  flesh,"  "ox. flesh,"  and 
"  swine's  flesh,"  of  their  English  subjects.  In  this  case, 
we  should  not  have  called  a  crayfish,  a  crayfish,  except 
for  the  Norman  conquest. 

On  the  other  hand,  if  "  crevik"  is  the   source  of  our 


THE  TECHNICAL   NAME   OF  THE  CRAYFISH.  13 

word,  it  may  have  come  to  us  straight  from  the  Angle 
and  Saxon  contingent  of  our  mixed  ancestry. 

As  to  the  origin  of  the  technical  name ;  dora/co's,  astakcs, 
was  the  name  hy  which  the  Greeks  knew  the  lobster ;  and 
it  has  been  handed  down  to  us  in  the  works  of  Aristotle, 
who  does  not  seem  to  have  taken  any  special  notice  of  the 
crayfish.  At  the  revival  of  learning,  the  eaily  naturalists 
noted  the  close  general  similarity  between  the  lobster  and 
the  crayfish ;  but,  as  the  latter  lives  in  fresh  water,  while 
the  former  is  a  marine  animal,  they  called  the  crayfish, 
in  their  Latin,  Astacus fluviatilis,  or  the  "river-lobster," 
by  way  of  distinction ;  and  this  nomenclature  was  re- 
tained until,  about  forty-five  years  ago,  an  eminent 
French  Naturalist,  M.  Milne-Edwards,  pointed  out  that 
there  are  far  more  extensive  differences  between  lobsters 
and  crayfish  than  had  been  supposed ;  and  that  it  would 
be  advisable  to  mark  the  distinctness  of  the  things  by 
a  corresponding  difference  in  their  names.  Leaving 
Astacus  for  the  crayfishes,  he  proposed  to  change  the 
technical  name  of  the  lobster  into  Homarus,  by  latin- 
ising the  old  French  name  "  Omar,"  or  "  Homar  "  (now 
Howard),  for  that  animal. 

At  the  present  time,  therefore,  while  the  recognised 
technical  name  of  the  crayfish  is  Astacus  fluviatilis,  that  of 
the  lobster  is  Homarus  vulgaris.  And  as  this  nomencla- 
I  lire  is  generally  received,  it  is  desirable  that  it  should  not 
be  altered ;  though  it  is  attended  by  the  inconvenience, 
that  Astacus,  as  we  now  employ  the  name,  does  not 


14       THE  NATURAL   HISTORY  OF  THE  COMMON   CRAYFISH, 

denote  that  which  the  Greeks,  ancient  and  modern, 
signify,  by  its  original,  astakos ;  and  does  signify 
something  quite  different. 

Finally,  as  to  why  it  is  needful  to  have  two  names 
for  the  same  thing,  one  vernacular,  and  one  technical. 
Many  people  imagine  that  scientific  terminology  is  a 
needless  burden  imposed  upon  the  novice,  and  ask  us 
why  we  cannot  be  content  with  plain  English.  In  reply, 
I  would  suggest  to  such  an  objector  to  open  a  conversation 
about  his  own  business  with  a  carpenter,  or  an  engineer, 
or,  still  better,  with  a  sailor,  and  try  how  far  plain 
English  will  go.  The  interview  will  not  have  lasted  long 
before  he  will  find  himself  lost  in  a  maze  of  unintelligible 
technicalities.  Every  calling  has  its  technical  termin- 
ology ;  and  every  artisan  uses  terms  of  art,  which  sound 
like  gibberish  to  those  who  know  nothing  of  the  art,  but 
are  exceedingly  convenient  to  those  who  practise  it. 

In  fact,  every  art  is  full  of  conceptions  which  are 
special  to  itself ;  and,  as  the  use  of  language  is  to  convey 
our  conceptions  to  one  another,  language  must  supply 
signs  for  those  conceptions.  There  are  two  ways  of 
doing  this :  either  existing  signs  may  be  combined  in 
loose  and  cumbrous  periphrases ;  or  new  signs,  having 
a  well-understood  and  definite  signification,  may  be  in- 
vented. The  practice  of  sensible  people  shows  the 
advantage  of  the  latter  course ;  and  here,  as  elsewhere, 
science  has  simply  followed  and  improved  upon  common 
sense. 


THE  USE  OF  THE  BINOMIAL  NOMENCLATIVE.         15 

Moreover,  while  English,  French,  German,  and  Italian 
artisans  are  under  no  particular  necessity  to  discuss 
the  processes  and  results  of  their  business  with  one 
another,  science  is  cosmopolitan,  and  the  difficulties  of 
the  study  of  Zoology  would  he  prodigiously  increased,  if 
Zoologists  of  different  nationalities  used  different  tech- 
nical terms  for  the  same  thing.  They  need  a  universal 
language ;  and  it  has  been  found  convenient  that  the  lan- 
guage shall  be  the  Latin  in  form,  and  Latin  or  Greek  in 
origin.  What  in  English  is  Crayfish,  is  Ecrevisse  in 
French;  Flusskrebs,  in  German;  Camnnro,  or  Gamlaro, 
or  Gamrnarello,  in  Italian  :  but  the  Zoologist  of  each 
nationality  knows  that,  in  the  scientific  works  of  all  the 
rest,  he  shall  find  what  he  wants  to  read  under  the  head 
of  Astacus  fluviatilis. 

But  granting  the  expediency  of  a  technical  name  for 
the  Crayfish,  why  should  that  name  be  double  ?  The 
reply  is  still,  practical  convenience.  If  there  are  ten 
children  of  one  family,  we  do  not  call  them  all  Smith, 
because  such  a  procedure  would  not  help  us  to  dis- 
tinguish one  from  the  other ;  nor  do  we  call  them 
simply  John,  James,  Peter,  William,  and  so  on,  for 
that  would  not  help  us  to  identify  them  as  of  one  family. 
So  we  give  them  all  two  names,  one  indicating  their 
close  relation,  and  the  other  their  separate  individuality 
— as  John  Smith,  James  Smith,  Peter  Smith,  William 
Smith,  &c.  The  same  thing  is  done  in  Zoology ;  only, 
iu  accordance  with  the  genius  of  the  Latin  language, 


16       THE  NATURAL   HISTORY  OF  THE  COMMON  CRAYFISH, 

we  put  the  Christian  name,  so  to  speak,  after  the  sur- 
name. 

There  are  a  number  of  kinds  of  Crayfish,  so  similar 
to  one  another  that  they  hear  the  common  surname  of 
Astacus.  One  kind,  by  way  of  distinction,  is  called 
fluviatile,  another  slender-handed,  another  Dauric,  from 
the  region  in  which  it  lives ;  and  these  double  names  are 
rendered  by — Astacus  fluviatilis,  Astacus  leptodactylus, 
and  A  stacus  dauricus ;  and  thus  we  have  a  nomenclature 
which  is  exceedingly  simple  in  principle,  and  free  from 
confusion  in  practice.  And  I  may  add  that,  the  less 
attention  is  paid  to  the  original  meaning  of  the  sub- 
stantive and  adjective  terms  of  this  binomial  nomen- 
clature, and  the  sooner  they  are  used  a^s  proper  names, 
the  better.  Very  good  reasons  for  using  a  term  may 
exist  when  it  is  first  invented,  which  lose  their  validity 
with  the  progress  of  knowledge.  Thus  Astacus  fluviatilis 
was  a  significant  name  so  long  as  we  knew  of  only  one 
kind  of  crayfish  ;  but  now  that  we  are  acquainted  with  a 
number  of  kinds,  all  of  which  inhabit  rivers,  it  is  meaning- 
less. Nevertheless,  as  changing  it  would  involve  endless 
confusion,  and  the  object  of  nomenclature  is  simply  to 
have  a  definite  name  for  a  definite  thing,  nobody  dreams 
of  proposing  to  alter  it. 

Having  learned  this  much  about  the  origin  of  the 
names  of  the  crayfish,  we  may  next  proceed  to  consider 
those  points  which  an  observant  Naturalist,  who  did  not 


THE  SKELETON   EXTERNAL   AND   CALCIFIED.  17 

care  to  go  far  beyond  the  surface  of  things,  would  find  to 
notice  in  the  animal  itself. 

Probably  the  most  conspicuous  peculiarity  of  the  cray- 
fish, to  any  one  who  is  familiar  only1  with  the  higher 
animals,  is  the  fact  that  the  hard  parts  of  the  body  are 
outside  and  the  soft  parts  inside ;  whereas  in  ourselves, 
and  in  the  ordinar}T  domestic  animals,  the  hard  parts,  or 
bones,  which  constitute  the  skeleton,  are  inside,  and  the 
soft  parts  clothe  them.  Hence,  while  our  hard  framework 
is  said  to  be  an  endoskeleton,  or  internal  skeleton ;  that 
of  the  crayfish  is  termed  an  exoskeleton,  or  external 
skeleton.  It  is  from  the  circumstance  that  the  body  of 
the  crayfishes  is  enveloped  in  this  hard  crust,  that 
the  name  of  Crustacea  is  applied  to  them,  along  with 
the  crabs,  shrimps,  and  other  such  animals.  Insects, 
spiders,  and  centipedes  have  also  a  hard  exoskeleton, 
but  it  is  usually  not  so  hard  and  thick  as  in  the 
Crustacea. 

If  a  piece  of  the  crayfish's  skeleton  is  placed  in  strong 
vinegar,  abundant  bubbles  of  carbonic  acid  gas  are  given 
off  from  it,  and  it  rapidly  becomes  converted  into  a  soft 
laminated  membrane,  while  the  solution  will  be  found  to 
contain  lime.  In  fact  the  exoskeleton  is  composed  of 
a  peculiar  animal  matter,  so  much  impregnated  with 
carbonate  and  phosphate  of  lime  that  it  becomes  dense 
and  hard. 

It  will  be  observed  that  the  body  of  the  crayfish  is 
naturally  marked  out  into  several  distinct  regions.  There 
3 


FIG.  2.— Aatacmflm-iatilis.— Dorsal  or  tergal  views  (nat.  size).  A,  male; 
B,  female  :— beg,  branchio-cardiac  groove,  which  marks  the  boun- 
dary between  the  pericardial  and  the  branchial  cavities  ?  eg,  cervical 
groove  ;  these  letters  are  placed  on  the  carapace  ;  r,  rostrum  ;  t,  t', 
the  two  divisions  of  the  telson  ;  1,  eye-stalks  ;  2,  antennules  ;  3, 
antennae  ;  20,  lateral  lobes  of  tail-fin  ;  xv-xx,  somites  of  the 
abdomen. 


THE    EXOSKELETON.  19 

is  a  firm  and  solid  front  part,  covered  by  a  large  con- 
tinuous shield,  which  is  called  the  carapace ;  and  a  jointed 
hind  part,  commonly  termed  the  tail  (fig.  2).  From 
the  perception  of  a  partially  real,  and  partially  fanciful, 
analogy  with  the  regions  into  which  the  body  is  divided 
in  the  higher  animals,  the  fore  part  is  termed  the  cepha- 
lo-thorax,  or  head  (cephalori)  and  chest  (thorax)  com- 
bined, while  the  hinder  part  receives  the  name  of 
abdomen. 

Now  the  exoskeleton  is  not  of  the  same  constitution 
throughout  these  regions.  The  abdomen,  for  example, 
is  composed  of  six  complete  hard  rings  (fig.  2,  xv-xx), 
and  a  terminal  flap,  on  the  under  side  of  which  the 
vent  (fig.  3,  a)  is  situated,  and  which  is  called  the  telson 
(fig.  2,  t,  t').  All  these  are  freely  moveable  upon  one 
another,  inasmuch  as  the  exoskeleton  which  connects 
them  is  not  calcified,  but  is,  for  the  most  part,  soft  and 
flexible,  like  the  hard  exoskeleton  when  the  lime  salts 
have  been  removed  by  acid.  The  mechanism  of  the  joints 
will  have  to  be  attentively  considered  by-and-by ;  it  is 
sufficient,  at  present,  to  remark  that,  wherever  a  joint 
exists,  it  is  produced  in  the  same  fashion,  by  the  exo- 
skeleton  remaining  soft  in  certain  regions  of  the  jointed 
part. 

The  carapace  is  not  jointed ;  but  a  transverse  groove  is 
observed  about  the  middle  of  it,  the  ends  of  which  run 
down  on  the  sides  and  then  turn  forwards  (figs.  1  and  2, 
eg).  This  is  called  the  cervical  groove,  and  it  marks  off 


20       THE  NATUEAL  HISTORY  OF  THE  COMMON   CRAYFISH, 

the  region  of  the  head,  in  front,  from  that  of  the  thorax 
behind. 

The  thorax  seems  at  first  not  to  be  jointed  at  all ;  but 
if  its  under,  or  what  is  better  called  its  sternal,  surface  is 
examined  carefully,  it  will  be  found  to  be  divided  into  as 
many  transverse  bands,  or  segments,  as  there  are  pairs  of 
legs  (fig.  3) ;  and,  moreover,  the  hindermost  of  these 
segments  is  not  firmly  united  with  the  rest,  but  can  be 
moved  backwards  and  forwards  through  a  small  space 
(fig.  3,  B  ;  xiv). 

Attached  to  the  sternal  side  of  every  ring  of  the  abdomen 
of  the  female  there  is  a  pair  of  limbs,  called  swimmerets. 
In  the  five  anterior  rings,  these  are  small  and  slender 
(fig.  3,  B;  15, 19)}  but  those  of  the  sixth  ring  are  very 
large,  and  each  ends  in  two  broad  plates  (20].  These 
two  plates  on  each  side,  with  the  telson  in  the  middle, 
constitute  the  flapper  of  the  crayfish,  by  the  aid  of  which 
it  executes  its  retrograde  swimming  movements.  The 
small  swimmerets  move  together  with  a  regular  swing, 
like  paddles,  and  probably  aid  in  propelling  the  animal 
forwards.  In  the  breeding  female  (B),  the  eggs  are 
attached  to  them ;  while,  in  the  male,  the  two  anterior 
pairs  (A ;  15,  16)  are  converted  into  the  peculiar  styles 
which  distinguish  that  sex. 

The  four  pairs  of  legs  which  are  employed  for  walking 
purposes,  are  divided  into  a  number  of  joints,  and  the 
foremost  two  pairs  are  terminated  by  double  claws, 
arranged  so  as  to  form  a  pincer,  whence  they  are  said  to 


FIG.  3.— Astacusfluviatilis.—  Ventral  or  sternal  views  (nat.  size).  A,  male  ;  B,  female  :— 
a,  vent ;  gg,  opening  of  the  green  gland ;  Ib,  labrum ;  mt,  metastoma  or  lower 
lip ;  od,  opening  of  the  oviduct ;  vd,  that  of  the  vas  deferens.  1,  eye-stalk ;  2, 
antennule  ;  3,  antenna  :  4,  mandible ;  S,  second  maxillipede ;  9,  third  or  external 
maxillipede  ;  10,  forceps  ;  11,  first  leg ;  1U,  fourth  leg ;  15,  16,  19,  20,  first,  second, 
fifth,  and  sixth  abdominal  appendages  :  x.,  XT.,  xiv  ,  sterna  of  the  fourth,  fifth, 
and  eighth  thoracic  somite  ;  xvi.,  sternum  of  the  second  abdominal  somite.  In  the 
male,  the  9th  to  the  14th  and  the  16th  to  the  19th  appendages  are  removed  on 
the  animal's  left  side  :  in  th?  female,  the  antenna  (with  the  exception  of  its  basal 
joint)  and  the  5th  to  the  14th  appendages  on  the  animal's  right  are  removed;  the 
eggs  also  are  shown  attached  to  the  swimmeruts  of  the  left  side  of  the  body. 


22       THE  NATURAL   HISTORY  OF  THE  COMMON  CRAYFISH, 

be  chelate.  The  two  hindermost  pairs,  on  the  other 
hand,  end  in  simple  claws. 

In  front  of  these  legs,  come  the  great  prehensile 
limbs  (10),  which  are  chelate,  like  those  which  im- 
mediately follow  them,  but  vastly  larger.  They  often 
receive  the  special  name  of  chelce ;  and  the  large  terminal 
joints  are  called  the  "  hand."  We  shall  escape  confusion 
if  we  call  these  limbs  the  forceps,  and  restrict  the  name 
of  chela  to  the  two  terminal  joints. 

All  the  limbs  hitherto  mentioned  subserve  locomotion 
and  prehension  in  various  degrees.  The  crayfish  swims 
by  the  help  of  its  abdomen,  and  the  hinder  pairs  of  ab- 
dominal limbs  ;  walks  by  means  of  the  four  hinder  pairs 
of  thoracic  limbs  ;  lays  hold  of  anything  to  fix  itself,  or 
to  assist  in  climbing,  by  the  two  chelate  anterior  pairs  of 
these  limbs,  which  are  also  employed  in  tearing  the  food 
seized  by  the  forceps  and  conveying  it  to  the  mouth ; 
while  it  seizes  its  prey  and  defends  itself  with  the  forceps. 
The  part  which  each  of  these  limbs  plays  is  termed  its 
function,  and  it  is  said  to  be  the  organ  of  that  function  ; 
so  that  all  these  limbs  may  be  said  to  be  organs  of  the 
functions  of  locomotion,  of  offence  and  defence. 

In  front  of  the  forceps,  there  is  a  pair  of  limbs  which 
have  a  different  character,  and  take  a  different  direction 
from  any  of  the  foregoing  (9).  These  limbs,  in  fact,  are 
turned  directly  forwards,  parallel  with  one  another,  and 
with  the  middle  line  of  the  body.  They  are  divided  into 
a  number  of  joints,  of  which  one  of  those  near  the  base 


THE   FOOT-JAWS   AND   THE   JAWS.  23 

is  longer  than  the  rest,  and  strongly  toothed  along  the 
inner  edge,  or  that  which  is  turned  towards  its  fellow. 
It  is  obvious  that  these  two  limbs  are  well  adapted  to 
crush  and  tear  whatever  conies  between  them,  and  they 
are,  in  fact,  jaws  or  organs  of  manducation.  At  the  same 
time,  it  will  be  noticed  that  they  retain  a  curiously  close 
general  resemblance  to  the  hinder  thoracic  legs ;  and 
hence,  for  distinction's  sake,  they  are  called  outer  foot- 
jaws,  or  external  maxillipedes. 

If  the  head  of  a  stout  pin  is  pushed  between  these 
external  maxillipedes,  it  will  be  found  that  it  passes 
without  any  difficulty  into  the  interior  of  the  body, 
through  the  mouth.  In  fact,  the  mouth  is  relatively 
rather  a  large  aperture ;  but  it  cannot  be  seen  without 
forcing  aside,  not  only  these  external  foot-jaws,  but  a 
number  of  other  limbs,  which  subserve  the  same  function 
of  manducation,  or  chewing  and  crushing  the  food.  We 
may  pass  by  the  organs  of  manducation,  for  the  present, 
with  the  remark  that  there  are  altogether  three  pairs  of 
maxillipedes,  followed  by  two  pairs  of  somewhat  differently 
formed  maxilla,  and  one  pair  of  very  stout  and  strong 
jaws,  which  are  termed  the  mandibles  (4).  All  these  jaws 
work  from  side  to  side,  in  contradistinction  to  the  jaws 
of  vertebrated  animals,  which  move  up  and  down.  In 
front  of,  and  above  the  mouth,  with  the  jaws  which 
cover  it,  are  seen  the  long  feelers,  which  are  called  the 
antenna  (3) ;  above,  and  in  front  of  them,  follow  the 
small  feelers,  or  antennules  (2) ;  and  over  them,  again,  lie 


24      THE  NATURAL   HISTORY  OF   THE   COMMON   CRAYFISH. 

the  eye  stalks  (1).     The  antennae  are  organs  of  touch 
the  antennules,  in  addition,  contain  the  organs  of  hear- 
ing ;  while,  at  the  ends  of  the  eyestalks,  are  the  organa 
of  vision. 

Thus  we  see  that  the  crayfish  has  a  jointed  and 
segmented  body,  the  rings  of  which  it  is  composed  being 
very  obvious  in  the  abdomen,  but  more  obscurely  trace- 
able elsewhere ;  that  it  has  no  fewer  than  twenty  pairs 
of  what  may  be  called  by  the  general  name  of  ap- 
pendages /  and  that  these  appendages  are  turned  to 
different  uses,  or  are  organs  of  different  functions,  in 
different  parts  of  the  body.  The  crayfish  is  obviously 
a  very  complicated  piece  of  living  machinery.  But  we 
have  not  yet  come  to  the  end  of  all  the  organs  that  may 
be  discovered  even  by  cursory  inspection.  Every  one 
who  has  eaten  a  boiled  crayfish,  or  a  lobster,  knows 
that  the  great  shield,  or  carapace,  is  very  easily  separated 
from  the  thorax  and  abdomen,  the  head  and  the  limbs 
which  belong  to  that  region  coming  away  with  the 
carapace.  The  reason  of  this  is  not  far  to  seek.  The 
lower  edges  of  that  part  ,of  the  carapace  which  belongs  to 
the  thorax  approach  the  bases  of  the  legs  pretty  closely, 
but  a  cleft-like  space  is  left;  and  this  cleft  extends 
forwards  to  the  sides  of  the  region  of  the  mouth,  and 
backwards  and  upwards,  between  the  hinder  margin  of 
the  carapace  and  the  sides  of  the  first  ring  of  the  abdo- 
men, which  are  partly  overlapped  by,  and  partly  overlap, 
that  margin.  If  the  blade  of  a  pair  of  scissors  is  care- 


THE   BRANCHIAL  CHAMBER  AND   THE  GILLS.  25 

fully  introduced  into  the  cleft  from  behind,  as  high  up 
as  it  will  go  without  tearing  anything,  and  a  cut  is  then 
made,  parallel  with  the  middle  line,  as  far  as  the  cervical 
groove,  and  thence  following  the  cervical  groove  to  the 
base  of  the  outer  foot-jaws,  a  large  flap  will  be  removed. 
This  flap  of  the  carapace  is  called  the  branchiostegite 
(fig.  1,  bg),  because  it  covers  the  gills  or  branchiae 
(fig.  4),  which  are  now  exposed.  They  have  the  appear- 
ance of  a  number  of  delicate  plumes,  which  take  a  direc- 
tion from  the  bases  of  the  legs  upwards  and  forwards 
behind,  upwards  and  backwards  in  front,  their  summits 
converging  towards  the  upper  end  of  the  cavity  in  which 
they  are  placed,  and  which  is  called  the  branchial 
chamber.  These  branchiae  are  the  respiratory  organs; 
and  they  perform  the  same  functions  as  the  gills  of  a 
fish,  to  which  they  present  some  similarity. 

If  the  gills  are  cleared  away,  it  is  seen  that  the  branchial 
cavity  is  bounded,  on  the  inner  side,  by  a  sloping  wall, 
formed  by  a  delicate,  but  more  or  less  calcified  layer  of 
the  exoskeleton,  which  constitutes  the  proper  outer  wall 
of  the  thorax.  At  the  upper  limit  of  the  branchial  cavity, 
the  layer  of  exoskeleton  is  very  thin,  and  turning  out- 
wards, is  continued  into  the  inner  wall  or  lining  of  the 
branchiostegite,  which  is  also  very  thin  (see  fig.  15,  p.  70). 

Thus  the  branchial  chamber  is  altogether  outside  the 
body,  to  which  it  stands  in  somewhat  the  same  relation 
as  the  space  between  the  flaps  of  a  man's  coat  and  hia 
waistcoat  would  do  to  the  part  of  the  body  enclosed  by  the 


pdb.Q 


7H/6.I3 


15 


14 


8  10 


«r7>.8    (17*6.9   pn>.!2     arb.  13          pit  13 

•" — '    y — ' — -" 


FIG.  4.—  Astacus  ftuvifttilis.—  In  A,  the  gills,  exposed  by  tlie  removal  of  the  branchio- 
stegite,  are  seen  in  their  natural  position  ;  in  B,  the,  podobranchiee  (see  p.  75)  are  re- 
moved, and  the  anterior  set  of  arthrobranehise  turned  downwards  (x  2)  :  1,  eye-stalk  ; 
2,  antennule  ;  3,  antenna  ;  It,  mandible  ;  6,  scaphognathite  ;  7,  iirst  maxillipede,  in  B 
the  epipodite,  to  which  the  line  points,  is  partly  removed  ;  8,  second  maxillipede  : 
9,  third  maxillipede  ;  10,  forceps  ;  Ik,  fourth  ambulatory  leg  ;  15,  first  abdominal 
appendage;  xv.,  first,  and  xvi.,  second  abdominal  somite;  arJ>.  8,  arb.  9,  arb.  13, 
the  posterior  arthrobranchia?  of  the  second  and  third  maxillipedes  and  of  the  third 
ambulatory  leg  ;  arb'.  U,  arb'.  13,  the  anterior arthrobranchise  of  the  third  maxiiliptde 
and  of  the  third  ambulatory  leg  ;  pbd.  S,  ]iodol)ranchia'  of  the  second  maxillijiede  ; 
pbd.  13,  that  of  the  third  ambulatory  leg  ;  )>1b.  1~',  i>lb.  1J,  the  two  rudimentary 
pleurobranchia! ;  plb.  lit,  the  functional  pleurobranchia  ;  r,  rostrum. 


THE  BREATHING  APPARATUS.  27 

waistcoat,  if  we  suppose  the  lining  of  the  flaps  to  be  made 
in  one  piece  with  the  sides  of  the  waistcoat.  Or  a  closei 
parallel  still  would  be  brought  about,  if  the  skin  of  a 
man's  back  were  loose  enough  to  be  pulled  out,  on  each 
side,  into  two  broad  flaps  covering  the  flanks. 

It  will  be  observed  that  the  branchial  chamber  is  open 
behind,  below,  and  in  front ;  and,  therefore,  that  the  water 
in  which  the  crayfish  habitually  lives  has  free  ingress 
and  egress.  Thus  the  air  dissolved  in  the  water  enables 
breathing  to  go  on,  just  as  it  does  in  fishes.  As  is  the 
case  with  many  fishes,  the  crayfish  breathes  very  well 
out  of  the  water,  if  kept  in  a  situation  sufficiently  cool 
and  moist  to  prevent  the  gills  from  drying  up ;  and 
thus  there  is  no  reason  why,  in  cool  and  damp  weather, 
the  crayfish  should  not  be  able  to  live  very  well  on  land, 
at  any  rate  among  moist  herbage,  though  whether 
our  common  crayfishes  do  make  such  terrestrial  excur- 
sions is  perhaps  doubtful.  We  shall  see,  by-and-by,  that 
there  are  some  exotic  crayfish  which  habitually  live  on 
land,  and  perish  if  they  are  long  submerged  in  water. 

With  respect  to  the  internal  structure  of  the  crayfish, 
there  are  some  points  which  cannot  escape  notice,  how- 
ever rough  the  process  of  examination  may  be. 

Thus,  when  the  carapace  is  removed  in  a  crayfish 
which  has  been  just  killed,  the  heart  is  seen  still 
pulsating.  It  is  an  organ  of  considerable  relative  size 
(fig.  5,  ft),  which  is  situated  immediately  beneath  the 


FfG.  5. — Astaciis  fuciatilis. — A  male  specimen,  with  the  roof  of  the 
carapace  and  the  terga  of  the  abdominal  somites  removed  to  show 
the  viscera  (nat.  size)  : — aa,  antennary  artery  ;  aff,  anterior  gastric 
muscles  ;  amm,  adductor  muscles  of  the  mandibles;  cs,  cardiac 
portion  of  the  stomach  :  ////,  green  glands  ;  7>,  heart  ;  Jiff,  hind  gut. 
or  large  intestine  ;  Lr,  liver  ;  oa,  ophthalmic  artery  ;  pg  posterior 
gastric  muscles  ;  saa,  superior  abdominal  artery  ;  f,  testis  ;  vd,  vas 
deferens. 


THE    "  CRABS'-EYES.' 


29 


Aiiddle  region  of  that  part  of  the  carapace  which  lies 
behind  the  cervical  groove  ;  or,  in  other  words,  in  the 
dorsal  region  of  the  thorax.  In  front  of  it,  and  therefore 
in  the  head,  is  a  large  rounded  sac,  the  stomach  (fig.  5, 
cs ;  fig.  6,  cs,  ps),  from  which  a  very  delicate  intestine 
(figs.  5  and  6,  %)  passes  straight  back  through  the  thorax 
and  abdomen  to  the  vent  (fig.  6,  a). 

JOT          If 


FIG.  6. — Aftaen»JtwriafiKt.—A.  longitudinal  vertical  section  of  the  ali- 
mentary canal,  with  the  outline  of  the  body  (nat.  size)  : — a,  vent;  ag, 
anterior  gastric  muscle  ;  Id,  entrance  of  left  bile  duct ;  eg,  cervical 
groove ;  c(f,  caecum  ;  cpv,  cardio-pyloric  valve  ;  cs,  cardiac  portion 
of  stomach :  the  circular  area  immediately  below  the  end  of  the 
line  from  cs  marks  the  position  of  the  gastrolith  of  the  left 
side ;  /iff,  hind-g-ut;  Ib,  labrum ;  It,  lateral  tooth  of  stomach  : 
m,  mouth  ;  /////,  mid-gut ;  nit,  median  tooth;  a, oesophagns ; pc, pro- 
cephalic  process ;  pg,  posterior  gastric  muscle  ;  p*t  pyloric  portion  of 
stomach  ;  /•,  annular  ridge,  marking  the  commencement  of  the 
hind-gut. 

In  summer,  there  are  commonly  to  be  found  at  the  sides 
of  the  stomach  two  lenticular  calcrreous  masses,  which 
are  known  as  "  crabs'-eyes,"  or  gastroliths,  and  were,  in 
old  times,  valued  in  medicine  as  sovereign  remedies  for  all 
sorts  of  disorders.  These  bodies  (fig.  7)  are  smooth  and 
flattened,  or  concave,  on  the  side  which  is  turned  towards 


SO       THE   NATUKAL   HISTORY   OF  THE  COMMON   CRAYFISH. 

the  cavity  of  the  stomach ;  while  the  opposite  side,  being 
convex  and  rough  with  irregular  prominences,  is  some- 
thing like  a  "  brain-stone  "  coral. 

Moreover,  when  the  stomach  is  laid  open,  three  large 


B 


FIG.   l.—Astacvs  fiumatilis. — A  gastrolith  ;   A,   from  above  ;  B,  from 
below  ;  C,  from  one  side  (all  x  5)  ;  D,  in  vertical  section  (  x  20). 

reddish  teeth  are  seen  to  project  conspicuously  into  its 
interior  (fig.  6,  It,  mt)  ;  so  that,  in  addition  to  its  six 
pairs  of  jaws,  the  crayfish  has  a  supplementary  crushing 
mill  in  its  stomach.  On  each  side  of  the  stomach,  there 
is  a  soft  yellow  or  brown  mass,  commonly  known  as  the 


THE  GROWTH  OF  THE  CRAYFISH.         31 

liver  (fig.  5,  Lr) ;  and,  in  the  breeding  season,  the 
ovaries  of  the  females,  or  organs  in  which  the  eggs  are 
formed,  are  very  conspicuous  from  the  dark-coloured 
eggs  which  they  contain,  and  which,  like  the  exoskeleton, 
turn  red  when  they  are  hoiled.  The  corresponding  part 
in  a  cooked  lobster  goes  by  the  name  of  the  "  coral." 

Beside  these  internal  structures,  the  most  noticeable 
are  the  large  masses  of  flesh,  or  muscle,  in  the  thorax 
and  abdomen,  and  in  the  pincers ;  which,  instead  of 
being  red,  as  in  most  of  the  higher  animals,  is  white. 
It  will  further  be  observed  that  the  blood,  which  flows 
readily  when  a  crayfish  is  wounded,  is  a  clear  fluid,  and 
is  either  almost  colourless,  or  of  a  very  pale  reddish  or 
neutral  tint.  Hence  the  older  Naturalists  thought  that 
the  crayfish  was  devoid  of  blood,  and  had  merely  a  sort 
of  ichor  in  place  of  it.  But  the  fluid  in  question  is  true 
blood  ;  and  if  it  is  received  into  a  vessel,  it  soon  forms  a 
soft,  but  firm,  gelatinous  clot. 

The  crayfish  grows  rapidly  in  youth,  but  enlarges  more 
and  more  slowly  as  age  advances.  The  young  animal  which 
has  just  left  the  egg  is  of  a  greyish  colour,  and  about 
one  quarter  of  an  inch  long.  By  the  end  of  the  year,  it 
may  have  reached  nearly  an  inch  and  a  half  in  length. 
Crayfishes  of  a  year  old  are,  on  an  average,  two  inches 
long  ;  at  two  years,  two  inches  and  four-fifths  ;  at  three 
years,  three  inches  and  a  half ;  at  four  years,  four  inches 
and  a  half  nearly  ;  and  at  five  years,  five  inches.  They 


32      THE  NATURAL   HISTORY    OF   THE   COMMON   CRAYFISH. 

go  on  growing  till,  in  exceptional  cases,  they  may  attain 
between  seven  inches  and  eight  inches  in  length;  but  at 
what  degree  of  longevity  this  unusual  dimension  is  reached 
w  uncertain.  It  seems  probable,  however,  that  the  life  of 
these  animals  may  be  prolonged  to  as  much  as  fifteen  or 
twenty  years.  They  appear  to  reach  maturity,  so  far  as 
the  power  of  reproduction  is  concerned,  in  their  fifth  or, 
more  usually,  their  sixth  year.  However,  I  have  seen 
a  female,  with  eggs  attached  under  the  abdomen,  only 
two  inches  long,  and  therefore,  probably,  in  her  second 
year.  The  males  are  commonly  larger  than  females  of 
the  same  age. 

The  hard  skeleton  of  a  crayfish,  once  formed,  is 
incapable  of  being  stretched,  nor  can  it  increase  by  in- 
terstitial addition  to  its  substance,  as  the  bone  of  one 
of  the  higher  animals  grows.  Hence  it  follows,  that  the 
enlargement  of  the  body,  which  actually  takes  place, 
involves  the  shedding  and  reproduction  of  its  invest- 
ment. This  might  be  effected  by  insensible  degrees,  and 
in  different  parts  of  the  body  at  different  times,  as  we 
shed  our  hair ;  but,  as  a  matter  of  fact,  it  occurs  periodi- 
cally and  universally,  somewhat  as  the  feathers  of  birds 
are  moulted.  The  whole  of  the  old  coat  of  the  body  is 
thrown  off  at  once,  and  suddenly ;  and  the  new  coat, 
which  has,  in  the  meanwhile,  been  formed  beneath 
the  old  one,  remains  soft  for  a  time,  and  allows  of  a 
rapid  increase  in  the  dimensions  of  the  body  before  it 


THE  SHEDDING  OF  THE  SKIN.  33 

hardens.  This  sort  of  moulting  is  what  is  technically 
termed  ecdysis,  or  exuviation.  It  is  commonly  spoken  of 
as  the  "  shedding  of  the  skin,"  and  there  is  no  harm  in 
using  this  phrase,  if  we  recollect  that  the  shed  coat  is  not 
the  skin,  in  the  proper  sense  of  the  word,  but  only  what 
is  termed  a  cuticular  layer,  which  is  secreted  upon  the 
outer  surface  of  the  true  integument.  The  cuticular 
skeleton  of  the  crayfish,  in  fact,  is  not  even  so  much  a 
part  of  the  skin  as  the  cast  of  a  snake,  or  as  our  own  nails. 
For  these  are  composed  of  coherent,  formed  parts  of  the 
epidermis ;  while  the  hard  investment  of  the  crayfish  con- 
tains no  such  formed  parts,  and  is  developed  on  the  out- 
side of  those  structures  which  answer  to  the  constituents 
of  the  epidermis  in  the  higher  animals.  Thus  the  cray- 
fish grows,  as  it  were,  by  starts ;  its  dimensions  remaining 
stationary  in  the  intervals  of  its  moults,  and  then  rapidly 
increasing  for  a  few  days,  while  the  new  exoskeleton  is 
in  the  course  of  formation. 

The  ecdysis  of  the  crayfish  was  first  thoroughly 
studied  a  century  and  a  half  ago,  by  one  of  the  most 
accurate  observers  who  ever  lived,  the  famous  Keaumur, 
and  the  following  account  of  this  very  curious  process  is 
given  nearly  in  his  words.* 

A  few   hours   before   the  process  of  exuviation  com- 

*  See  Re'Mimur's  two  Memoirs,  *'  Sur  les  diverses  reproductions  qui 
ee  font  dans  les  ecrevisses,  les  omars,  les  crabes,  etc.,"  "  Histoire  de 
I'Academie  royale  des  Sciences,"  annee  3712  ;  and  "Additions  attx  ob- 
servations sur  la  mue  des  dcrevisses  donnees  dans  les  Me"moires  de  1712." 
Ibid.  1718. 

4 


34       THE  NATURAL  HISTORY   OF  THE  COMMON   CRAYFISH, 

mences,  the  crayfish  rubs  its  limbs  one  against  the 
other,  and,  without  changing  its  place,  moves  each 
separately,  throws  itself  on  its  back,  bends  its  tail, 
and  then  stretches  it  out  again,  at  the  same  time  vibrat* 
ing  its  antennae.  By  these  movements,  it  gives  the 
various  parts  a  little  play  in  their  loosened  sheaths. 
After  these  preparatory  steps,  the  crayfish  appears  to 
become  distended;  in  all  probability,  in  consequence  of 
the  commencing  retraction  of  the  limbs  into  the  interior 
of  the  exoskeleton  of  the  body.  In  fact,  it  has  been 
remarked,  that  if,  at  this  period,  the  extremity  of  one  of 
the  great  claws  is  broken  off,  it  will  be  found  empty, 
the  contained  soft  parts  being  retracted  as  far  as  the 
second  joint.  The  soft  membranous  part  of  the  exo- 
skeleton, which  connects  the  hinder  end  of  the  carapace 
with  the  first  ring  of  the  abdomen,  gives  way,  and  the 
body,  covered  with  the  new  soft  integument,  protrudes  ; 
its  dark  brown  colour  rendering  it  easily  distinguishable 
from  the  greenish-brown  old  integument. 

Having  got  thus  far,  the  crayfish  rests  for  a  while,  and 
then  the  agitation  of  the  limbs  and  body  recommences. 
The  carapace  is  forced  upwards  and  forwards  by  the  pro- 
trusion of  the  body,  and  remains  attached  only  in  the 
region  of  the  mouth.  The  head  is  next  drawn  backwards, 
while  the  eyes  and  its  other  appendages  are  extracted  from 
their  old  investment.  Next  the  legs  are  pulled  out,  either 
one  at  a  time,  or  those  of  one,  or  both,  sides  together. 
Sometimes  a  limb  gives  way  and  is  left  IHund  in  its  sheath. 


THE  SHEDDING  OF  THE  SKIN.          35 

The  operation  is  facilitated  by  the  splitting  of  the  old 
integument  of  the  limb  along  one  side  longitudinally. 

When  the  legs  are  disengaged,  the  animal  draws  itg 
head  and  limbs  completely  out  of  their  former  covering ; 
and,  with  a  sudden  spring  forward,  while  it  extends  its 
abdomen,  it  extracts  the  latter,  and  leaves  its  old  skele- 
ton behind.  The  carapace  falls  back  into  its  ordinary 
position,  and  the  longitudinal  fissures  of  the  sheaths  of 
the  limbs  close  up  so  accurately,  that  the  shed  integu- 
ment has  just  the  appearance  the  animal  had  when  the 
exuviation  commenced.  The  cast  exoskeleton  is  so  like 
the  crayfish  itself,  when  the  latter  is  at  rest,  that,  except 
for  the  brighter  colour  of  the  latter,  the  two  cannot  be 
distinguished. 

After  exuviation,  the  owner  of  the  cast  skin,  ex- 
hausted by  its  violent  struggles,  which  are  not  unfre- 
quently  fatal,  lies  in  a  prostrate  condition.  Instead  of 
being  covered  by  a  hard  shell,  its  integument  is  soft  and 
flabby,  like  wet  paper ;  though  Reaumur  remarks,  that 
if  a  crayfish  is  handled  immediately  after  exuviation,  its 
body  feels  hard ;  and  he  ascribes  this  to  the  violent  con- 
traction which  its  muscles  have  undergone,  leaving  them 
in  a  state  of  cramp.  In  the  absence  of  the  hard  skeleton, 
however,  there  is  nothing  to  bring  the  contracted  muscles 
at  once  back  into  position,  and  it  must  be  some  time 
before  the  pressure  of  the  internal  fluids  is  so  distributed 
as  to  stretch  them  out. 

When  the  process  of  exuviation  has  proceeded  so  far 


56       THE  NATURAL   HISTORY   OF  THE   COMMON   CRAYFISH. 

that  the  carapace  is  raised,  nothing  stops  the  crayfish 
from  continuing  its  struggles.  If  taken  out  of  the  water 
in  this  condition,  they  go  on  moulting  in  the  hand,  and 
even  pressure  on  their  bodies  will  not  arrest  their  efforts, 

The  length  of  time  occupied  from  the  first  giving  way 
of  the  integuments  to  the  final  emergence  of  the  animal, 
varies  with  its  vigour,  and  the  conditions  under  which  it 
is  placed,  from  ten  minutes  to  several  hours.  The 
chitinous  lining  of  the  stomach,  with  its  teeth,  and  the 
"  crabs'-eyes,"  are  shed  along  with  the  rest  of  the  cuti- 
cular  exoskeleton  ;  but  they  are  broken  up  and  dissolved 
in  the  stomach. 

The  new  integuments  of  the  crayfish  remain  soft  for 
a  period  which  varies  from  one  to  three  days  ;  and  it  is 
a  curious  fact,  that  the  animal  appears  to  be  quite  awn  re 
of  its  helplessness,  and  governs  itself  accordingly. 

An  observant  naturalist  says  :  "I  once  had  a  do- 
mesticated crayfish  (Astacus  fluviatilis) ,  which  I  kept  hi 
a  glass  pan,  in  water,  not  more  than  an  inch  and  a  half 
deep,  previous  experiment  having  shown  that  in  deeper 
water,  probably  from  want  of  sufficient  aeration,  this 
animal  would  not  live  long.  By  degrees  my  prisoner 
became  very  bold,  and  when  I  held  my  fingers  at  the 
edge  of  the  vessel,  he  assailed  them  with  promptness  and 
energy.  About  a  year  after  I  had  him,  I  perceived,  as  I 
thought,  a  second  crayfish  with  him.  On  examination, 
I  found  it  to  be  his  old  coat,  which  he  had  left  in  a  most 
perfect  state.  My  friend  had  now  lost  his  heroism,  and 


THE   REPRODUCTION   OF  LIMBS.  37 

fluttered  about  in  the  greatest  agitation.  He  was  quite 
soft ;  and  every  time  I  entered  the  room  during  the  next 
t\vo  da\  s,  he  exhibited  the  wildest  terror.  On  the  third, 
lie  appeared  to  gain  confidence,  and  ventured  to  use  his 
Dippers,  though  with  some  timidity,  and  he  was  not  yet 
quite  so  hard  as  he  had  been.  In  about  a  week,  how- 
ever, he  became  bolder  than  ever ;  his  weapons  were 
sharper,  and  he  appeared  stronger,  and  a  nip  from  him 
was  no  joke.  He  lived  in  all  about  two  years,  during 
which  time  his  food  was  a  very  few  worms  at  very  uncer- 
tain times  ;  perhaps  he  did  not  get  fifty  altogether/'  * 

It  would  appear,  from  the  best  observations  that  have 
yet  been  made,  that  the  young  crayfish  exuviate  two  or 
three  times  in  the  course  of  the  first  year ;  and  that, 
afterwards,  the  process  is  annual,  and  takes  place  usually 
about  midsummer.  There  is  reason  to  suppose  that  very 
old  crayfish  do  not  exuviate  every  year. 

It  has  been  stated  that,  in  the  course  of  its  violent 
efforts  to  extract  its  limbs  from  the  cast-off  exoskeleton, 
the  crayfish  sometimes  loses  one  or  other  of  them ;  the 
limb  giving  way,  and  the  greater  part,  or  the  whole,  of  it 
remaining  in  the  exuviae.  But  it  is  not  only  in  this  way 
that  crayfishes  part  with  their  limbs.  At  all  times,  if  the 
animal  is  held  by  one  of  its  pincers,  so  that  it  cannot 
get  away,  it  is  apt  to  solve  the  difficulty  by  casting  off 

*  Th3  late  Mr.  Robert  Ball,  of  Dublin,  in  Bell's  "  British  Crustacea," 
p.  239. 


38       THE   NATURAL  HISTORY  OP  THE   COMMON   CRAYFISH. 

the  limb,  which  remains  in  the  hand  of  the  captor,  while 
the  crayfish  escapes.  This  voluntary  amputation  is  always 
effected  at  the  same  place ;  namely,  where  the  limb  is 
slenderest,  just  beyond  the  articulation  which  unites  the 
basal  joint  with  the  next.  The  other  limbs  also  readily 
part  at  the  joints ;  and  it  is  very  common  to  meet  with 
crayfish  which  have  undergone  such  mutilation.  But 
the  injury  thus  inflicted  is  not  permanent,  as  these 
animals  possess  the  power  of  reproducing  lost  parts  to 
a  marvellous  extent,  whether  the  loss  has  been  inflicted 
by  artificial  amputation,  or  voluntarily. 

Crayfishes,  like  all  the  Crustacea,  bleed  very  freely  when 
wounded ;  and  if  one  of  the  large  joints  of  a  leg  is  cut 
through,  or  if  the  animal's  body  is  injured,  it  is  very  likely 
to  die  rapidly  from  the  ensuing  haemorrhage.  A  cray- 
fish thus  wounded,  however,  commonly  throws  off  the 
limb  at  the  next  articulation,  where  the  cavity  of  the 
limb  is  less  patent,  and  its  sides  more  readily  fall 
together ;  and,  as  we  have  seen,  the  pincers  are  usually 
cast  off  at  their  narrowest  point.  When  such  amputation 
has  taken  place,  a  crust,  probably  formed  of  coagulated 
blood,  rapidly  forms  over  the  surface  of  the  stump ;  and, 
eventually,  it  becomes  covered  with  a  cuticle.  Beneath 
this,  after  a  time,  a  sort  of  bud  grows  out  from  the 
centre  of  the  surface  of  the  stump,  and  gradually  takes 
on  the  form  of  as  much  of  the  limb  as  has  been  removed. 
At  the  next  ecdysis,  the  covering  cuticle  is  thrown  off 
along  with  the  rest  of  the  exoskeleton ;  while  the  rudi- 


THE   REPRODUCTION  OF  THE  SPECIES.  39 

mentary  limb  straightens  out/  and,  though  very  small, 
acquires  all  the  organization  appropriate  to  that  limb. 
At  every  moult  it  grows ;  but,  it  is  only  after  a  long  time 
that  it  acquires  nearly  the  size  of  its  uninjured  and  oldei 
fellow.  Hence,  it  not  unfrequently  happens,  that  crayfish 
are  found  with  pincers  and  other  limbs,  which,  though 
alike  useful  and  anatomically  complete,  are  very  unequal 
in  size. 

Injuries  inflicted  while  the  crayfish  are  soft  after 
moulting,  are  apt  to  produce  abnormal  growths  of  the 
part  affected;  and  these  may  be  perpetuated,  and  give 
rise  to  various  monstrosities,  in  the  pincers  and  in  other 
parts  of  the  body. 

In  the  reproduction  of  their  kind  by  means  of  eggs  the 
co-operation  of  the  males  with  the  females  is  necessary. 
On  the  basal  joint  of  the  hindermost  pair  of  legs  of  the  male 
a  small  aperture  is  to  be  seen  (fig.  3,  A;  vd).  In  these,  the 
ducts  of  the  apparatus  in  which  the  fecundating  substance 
is  formed  terminate.  The  fecundating  material  itself  is  a 
thickish  fluid,  which  sets  into  a  white  solid  after  extru- 
sion. The  male  deposits  this  substance  on  the  thorax 
of  the  female,  between  the  bases  of  the  hindermost  pairs 
of  thoracic  limbs. 

The  eggs  formed  in  the  ovary  are  conducted  to  apertures, 
which  are  situated  on  the  bases  of  the  last  pair  of  ambula- 
tory legs  but  two,  that  is,  in  the  hinder  of  the  two  pair 
which  are  provided  with  chelate  extremities  (fig.  3,  B ;  od). 


tO       THE   NATURAL   HISTORY   OF   THE   COMMON   CRAYFISH. 

After  the  female  has  received  the  deposit  of  the 
spermatic  matter  of  the  male,  she  retires  to  a  burrow, 
in  the  manner  already  stated,  and  then  the  process  of 
laying  the  eggs  commences.  These,  as  they  leave  the 
apertures  of  the  oviducts,  are  coated  with  a  viscid  matter, 
which  is  readily  drawn  out  into  a  short  thread.  The 
end  of  the  thread  attaches  itself  to  one  of  the  long  hairs, 
with  which  the  swimmerets  are  fringed,  and  as  the  viscid 
matter  rapidly  hardens,  the  egg  thus  becomes  attached 
to  the  limb  by  a  stalk.  The  operation  is  repeated,  until 
sometimes  a  couple  of  hundred  eggs  are  thus  glued  on 
to  the  swimmerets.  Partaking  in  the  movements  of  the 
swimmerets,  they  are  washed  backwards  and  forwards  in 
the  water,  and  thus  aerated  and  kept  free  of  impurities  ; 
while  the  young  crayfish  is  formed  much  in  the  same 
way  as  the  chick  is  formed  in  a  hen's  egg. 

The  process  of  development,  however,  is  very  slow, 
as  it  6ccupies  the  whole  winter.  In  late  spring-time,  or 
early  summer,  the  young  burst  the  thin  shell  of  the 
egg,  and,  when  they  are  hatched,  present  a  general  re- 
semblance to  their  parents.  This  is  very  unlike  what 
takes  place  in  crabs  and  lobsters,  in  which  the  young 
leave  the  egg  in  a  condition  very  different  from  the 
parent,  and  undergo  a  remarkable  metamorphosis  before 
they  attain  their  proper  form. 

For  some  time  after  they  are  hatched,  the  young  hold 
on  to  the  swimmerets  of  the  mother,  and  are  carried 
about,  protected  by  her  abdomen,  as  in  a  kind  of  nursery. 


NEWLY-HATCHED   CRAYFISHES.  41 

That   most  careful   naturalist,  Boesel  von  Rosenhof, 
says  of  the  young,  when  just  hatched  : — 

"  At  this  time  they  are  quite  transparent ;  and  when 


FIG.  8. — Astaciu  fluviatttig. — A,  two  recently  hatched  crayfish  attached 
.  to  one  of  the  swiminerets  of  the  mother  ( x  4).   pr,  protopodite  ; 
en,  endopodite  :  and  ex,  exopodite  of  the  swimmeret ;  ec,  ruptured 
egg-cases.    B,  chela  of  a  recently  hatched  crayfish  (  x  10). 

such  a  crayfish    [a    female   with   young]   is  brought  to 
table,  it  looks  quite  disgusting  to  those  who  do  not  know 


42       THE   NATURAL  HISTORY   OF  THE   COMMON   CRAYFISH. 

what  the  young  are ;  but  if  we  examine  it  more  closely, 
especially  with  a  magnifying-glass,  we  see  with  pleasure 
that  the  little  crayfish  are  already  perfect,  and  resemble 
the  large  one  in  all  respects.  When  the  mother  of  these 
little  crayfish,  after  they  have  begun  to  be  active,  is  quiet 
for  a  while,  they  leave  her  and  creep  about,  a  short  way 
off.  But,  if  they  spy  the  least  sign  of  danger,  or  there  is 
any  unusual  movement  in  the  water,  it  seems  as  if  the 
mother  recalled  them  by  a  signal ;  for  they  all  at  once 
swiftly  return  under  her  tail,  and  gather  into  a  cluster, 
and  the  mother  hies  to  a  place  of  safety  with  them,  as 
quickly  as  she  can.  A  few  days  later,  however,  they 
gradually  forsake  her."  * 

Fishermen  declare  that  "  Hen  Lobsters  "  protect  their 
young  in  a  similar  manner.!  Jonston,J  who  wrote  in 
the  middle  of  the  seventeenth  century,  says  that  the  little 
crayfish  are  often  to  be  seen  adhering  to  the  tail  of  the 
mother.  Roesel's  observations  imply  the  same  thing ; 
but  he  does  not  describe  the  exact  mode  of  adherence, 
and  I  can  find  no  observations  on  the  subject  in  the 
works  of  later  writers. 

It  has  been  seen  that  the  eggs  are  attached  to  the 
swimmerets  by  a  viscid  substance,  which  is,  as  it  were, 
smeared  over  them  and  the  hairs  with  which  they  are 

•  "Der  Monat'ich-herausgegeben  Insecten  Belustigung."  Drittei 
rheil,p.  336.  1755. 

f  Bell's  "  British  Crustacea,"  p.  249. 

£  "Joannis  Jonstoni  Historiae  naturalis  de  Piscibus  et  Cetis  Libri 
quinque.  Tomus  IV.  *  De  Cammaro  seu  Astaco  fluviatili.'" 


NEWLY-HATCHED   CRAYFISHES.  43 

fringed,  and  is  continued  by  longer  or  shorter  thread-like 
pedicles  into  the  coat  of  the  same  material  which  invests 
each  egg.  It  very  soon  hardens,  and  then  becomes  very 
6rm  and  elastic. 

When  the  young  crayfish  is  ready  to  be  hatched,  the  egg 
case  splits  into  two  moieties,  which  remain  attached,  like  a 
pair  of  watch  glasses,  to  the  free  end  of  the  pedicle  of  the 
egg  (fig.  8,  A ;  ec).  The  young  animal,  though  very  similar 
to  the  parent,  does  not  quite  "resemble  it  in  all  respects,'* 
as  Roesel  says.  For  not  only  are  the  first  and  the  last 
pairs  of  abdominal  limbs  wanting,  while  the  telson  is  very 
different  from  that  of  the  adult ;  but  the  ends  of  the  great 
chelae  are  sharply  pointed  and  bent  down  into  abruptly  in- 
curved hooks,  which  overlap  when  the  chelaB  are  shut  (fig.  8, 
B).  Hence,  when  the  chelae  have  closed  upon  anything  soft 
enough  to  allow  of  the  imbedding  of  these  hooks,  it  is 
very  difficult,  if  not  impossible,  to  open  them  again. 

Immediately  the  young  are  set  free,  they  must  instinc- 
tively bury  the  ends  of  their  forceps  in  the  hardened 
egg-glue  which  is  smeared  over  the  swirninerets,  for  they 
are  all  found  to  be  holding  on  in  this  manner.  They 
exhibit  very  little  movement,  and  they  bear  rough 
shaking  or  handling  without  becoming  detached  ;  in 
consequence,  I  suppose,  of  the  interlocking  of  the  hooked 
ends  of  the  chelae  imbedded  in  the  egg-glue. 

Even  after  the  female  has  been  plunged  into  alcohol, 
the  young  remain  attached.  I  have  had  a  female,  with 
young  affixed  in  this  manner,  under  observation  for  five 


44       THE  NATURAL   HISTORY  OF  THE  COMMON   CRAYFISH. 

days,  but  none  of  them  showed  any  signs  of  detaching 
themselves  ;  and  I  am  inclined  to  think  that  they  are 
set  free  only  at  the  first  moult.  After  this,  it  would 
appear  that  the  adhesion  to  the  parent  is  only  temporary. 

The  walking  legs  are  also  hooked  at  their  extremities, 
but  they  play  a  less  important  part  in  fixing  the  young 
to  the  parent,  and  seem  to  be  always  capable  of  loosing 
their  hold. 

I  find  the  young  of  a  Mexican  crayfish  (Cambarus)  to  be 
attached  in  the  same  manner  as  those  of  the  English 
crayfish;  but,  according  to  Mr.  Wood-Mason's  recent 
observations,  the  young  of  the  New  Zealand  crayfishes 
fix  themselves  to  the  swimmerets  of  the  parent  by  the 
hooked  ends  of  their  hinder  ambulatory  limbs. 

Crayfishes,  in  every  respect  similar  to  those  found 
in  our  English  rivers,  that  is  to  say,  of  the  species 
Astacus  fluviatilis,  are  met  with  in  Ireland,  and  on  the 
Continent,  as  far  south  as  Italy  and  northern  Greece ; 
as  far  east  as  western  Russia ;  and  as  far  north  as  the 
shores  of  the  Baltic.  They  are  not  known  to  occur  in 
Scotland  ;  in  Spain,  except  about  Barcelona,  they  are 
either  rare,  or  have  remained  unnoticed. 

There  is,  at  present,  no  proof  of  the  occurrence  of 
Astacus  fluviatilis  in  the  fossil  state. 

Curious  myths  have  gathered  about  crayfishes,  as 
about  other  animals.  At  one  time  "  crabs'-eyes  "  were 


CRAYFISHES  AND  PIGS.  45 

collected  in  vast  numbers,  and  sold  for  medicinal 
purposes  as  a  remedy  against  the  stone,  among  other 
diseases.  Their  real  utility,  inasmuch  as  they  consist 
almost  entirely  of  carbonate  of  lime,  with  a  little  phos- 
phate of  lime  and  animal  matter,  is  much  the  same  as 
that  of  chalk,  or  carbonate  of  magnesia.  It  was,  for- 
merly, a  current  belief  that  crayfishes  grow  poor  at  the 
time  of  new  moon,  and  fat  at  that  of  full  moon ;  and, 
perhaps,  there  may  be  some  foundation  for  the  notion, 
considering  the  nocturnal  habits  of  the  animals.  Van 
Helmont,  a  great  dealer  in  wonders,  is  responsible  for 
the  story  that,  in  Brandenburg,  where  there  is  a  grsat 
abundance  of  crayfishes,  the  dealers  were  obliged  to 
transport  them  to  market  by  night,  lest  a  pig  should 
run  under  the  cart.  For  if  such  a  misfortune  should 
happen,  every  crayfish  would  be  found  dead  in  the 
morning  :  "  Tarn  exitialis  est  porcus  cancro."  Another 
author  improves  the  story,  by  declaring  that  the  steam 
of  a  pig-stye,  or  of  a  herd  of  swine,  is  instantaneously 
fatal  to  crayfish.  On  the  other  hand,  the  smell  of 
putrifying  crayfish,  which  is  undoubtedly  of  the  strongest, 
was  said  to  drive  even  moles  out  of  their  burrows. 


CHAPTER  H. 

THE  PHYSIOLOGY  OF  THE  CRAYFISH.  THE  MECHANISM  Bl 
WHICH  THE  PARTS  OF  THE  LIVING  ENGINE  ARE  SUPPLIED 
WITH  THE  MATERIALS  NECESSARY  FOR  THEIR  MAIN- 
TENANCE AND  GROWTH. 

AN  analysis  of  such  a  sketch  of  the  "  Natural  History 
of  the  Grayish"  as  is  given  in  the  preceding  chapter, 
shows  that  it  provides  brief  and  general  answers  to  three 
questions.  First,  what  is  the  form  and  structure  of  the 
animal,  not  only  when  adult,  but  at  different  stages  of 
its  growth  ?  Secondly,  what  are  the  various  actions  of 
which  it  is  capable  ?  Thirdly,  where  is  it  found  ?  If  we 
carry  our  investigations  further,  in  such  a  manner  as  to 
give  the  fullest  attainable  answers  to  these  questions, 
the  knowledge  thus  acquired,  in  the  case  of  the  first 
question,  is  termed  the  Morphology  of  the  crayfish; 
in  the  case  of  the  second  question,  it  constitutes  the 
Physiology  of  the  animal ;  while  tbe  answer  to  the  third 
question  would  represent  what  we  know  of  its  Distribu- 
tion or  Chorology.  There  remains  a  fourth  problem, 
which  can  hardly  be  regarded  as  seriously  under  dis- 
cussion, so  long  as  knowledge  has  advanced  no  further 
than  the  Natural  History  stage ;  the  question,  namely, 


TELEOLOGY  AND  PHYSIOLOGY.  47 

how  all  these  facts  comprised  under  Morphology,  Physi- 
ology, and  Chorology  have  come  to  be  what  they  are ; 
and  the  attempt  to  solve  this  problem  leads  us  to  the 
crown  of  Biological  effort,  ^Etiology.  When  it  supplies 
answers  to  all  the  questions  which  fall  under  these  four 
heads,  the  Zoology  of  Crayfish  will  have  said  its  last 
word. 

As  it  matters  little  in  what  order  we  take  the  first  three 
questions,  in  expanding  Natural  History  into  Zoology, 
we  may  as  well  follow  that  which  accords  with  the  history 
of  science.  After  men  acquired  a  rough  and  general 
knowledge  of  the  animals  about  them,  the  next  thing  which 
engaged  their  interest  was  the  discovery  in  these  animals 
of  arrangements  by  which  results,  of  a  kind  similar  to 
those  which  their  own  ingenuity  effects  through  mechanical 
contrivances,  are  brought  about.  They  observed  that 
animals  perform  various  actions  ;  and,  when  they  looked 
into  the  disposition  and  the  powers  of  the  parts  by  which 
these  actions  are  performed,  they  found  that  these  parts 
presented  the  characters  of  an  apparatus,  or  piece  of 
mechanism,  the  action  of  which  could  be  deduced  from 
the  properties  and  connections  of  its  constituents,  just 
as  the  striking  of  a  clock  can  be  deduced  from  the 
properties  and  connections  of  its  weights  and  wheels. 

Under  one  aspect,  the  result  of  the  search  after  the 
rationale  of  animal  structure  thus  set  afoot  is  Teleology; 
or  the  doctrine  of  adaptation  to  purpose.  Under  another 


48        THE   PHYSIOLOGY  OF  THE   COMMON   CRAYFISH. 

aspect,  it  is  Physiology ;  so  far  as  Physiology  consists  in 
the  elucidation  of  complex  vital  phenomena  by  deduction 
from  the  established  truths  of  Physics  and  Chemistry,  or 
from  the  elementary  properties  of  living  matter. 

We  have  seen  that  the  crayfish  is  a  voracious  and 
indiscriminate  feeder ;  and  we  shall  be  safe  in  assuming 
that,  if  duly  supplied  with  nourishment,  a  full-grown 
crayfish  will  consume  several  times  its  own  weight  of 
food  in  the  course  of  the  year.  Nevertheless,  the  increase 
of  the  animal's  weight  at  the  end  of  that  time  is,  at  most, 
a  small  fraction  of  its  total  weight ;  whence  it  is  quite 
clear,  that  a  very  large  proportion  of  the  food  taken  into 
the  body  must,  in  some  shape  or  other,  leave  it  again. 
In  the  course  of  the  same  period,  the  crayfish  absorbs  a 
very  considerable  quantity  of  oxygen,  supplied  by  the 
atmosphere  to  the  water  which  it  inhabits  ;  while  it  gives 
out,  into  that  water,  a  large  amount  of  carbonic  acid,  and 
a  larger  or  smaller  quantity  of  nitrogenous  and  other  ex- 
crementitious  matters.  From  this  point  of  view,  the 
crayfish  may  be  regarded  as  a  kind  of  chemical  manu- 
factory, supplied  with  certain  alimentary  raw  materials, 
which  it  works  up,  transforms,  and  gives  out  in  other 
shapes.  And  the  first  physiological  problem  which  offers 
itself  to  us  is  the  mode  of  operation  of  the  apparatus 
contained  in  this  factory,  and  the  extent  to  which  the 
products  of  its  activity  are  to  be  accounted  for  by 
reasoning  from  known  physical  and  chemical  principles. 


THE  PKOCESS  OF  FEEDING.  49 

We  have  learned  that  the  food  of  the  crayfish  is  made 
op  of  very  diverse  substances,  both  animal  and  vegetable ; 
but,  so  far  as  they  are  competent  to  nourish  the  animal 
permanently,  these  matters  all  agree  in  containing  a 
peculiar  nitrogenous  body,  termed  protein,  under  one  of  its 
many  forms,  such  as  albumen,  fibrin,  and  the  like.  With 
this  may  be  associated  fatty  matters,  starchy  and  sac- 
charine  bodies,  and  various  earthy  salts.  And  these, 
which  are  the  essential  constituents  of  the  food,  may  be, 
and  usually  are,  largely  mixed  up  with  other  substances, 
guch  as  wood,  in  the  case  of  vegetable  food,  or  skeletal 
and  fibrous  parts,  in  the  case  of  animal  prey,  which  are 
of  little  or  110  utility  to  the  crayfish. 

The  first  step  in  the  process  of  feeding,  therefore,  is 
to  reduce  the  food  to  such  a  state,  that  the  separation 
of  its  nutritive  parts,  or  those  which  can  be  turned  to 
account,  from  its  innutritions,  or  useless,  constituents, 
may  be  facilitated.  And  this  preliminary  operation  is 
the  subdivision  of  the  food  into  morsels  of  a  convenient 
size  for  introduction  into  that  part  of  the  machinery  in 
which  the  extraction  of  the  useful  products  i*  performed. 

The  food  may  be  seized  by  the  pinceis,  or  by  the 
anterior  chelate  ambulatory  limbs ;  and,  in  the  former 
case,  it  is  usually,  if  not  always,  transferred  to  the  first, 
or  second,  or  both  of  the  anterior  pairs  of  ambulatory 
limbs.  These  grasp  the  food,  and,  tearing  it  into 
pieces  of  the  proper  dimensions,  thrust  them  between 

the  external  maxillipedes,  which  are,  at  the  same  time, 
5 


50        THE   PHYSIOLOGY   OF  THE  COMMON   CRAYFISH. 

worked  rapidly  to  and  fro  sideways,  so  as  to  bring  their 
toothed  edges  to  be^r  upon  the  morsel.  The  other  five 
pairs  of  jaws  are  no  less  active,  and  they  thus  crush  and 
divide  the  food  brought  to  them,  as  it  is  passed  between 
their  toothed  edges  to  the  opening  of  the  mouth. 

As  the  alimentary  canal  stretches  from  the  mouth, 
at  one  end,  to  the  vent  at  the  other,  and,  at  each  of 
these  limits,  is  continuous  with  the  wall  of  the  body, 
we  may  conceive  the  whole  crayfish  to  be  a  hollow 
cylinder,  the  cavity  of  which  is  everywhere  closed,  though 
it  is  traversed  by  a  tube,  open  at  each  end  (fig.  6). 
The  shut  cavity  between  the  tube  and  the  walls  of  the 
cylinder  may  be  termed  the  perivisceral  cavity ;  and  it  is 
so  much  filled  up  by  the  various  organs,  which  are  inter- 
posed between  the  alimentary  canal  and  the  body  wall, 
that  all  that  is  left  of  it  is  represented  by  a  system  of 
irregular  channels,  which  are  filled  with  blood,  and  are 
termed  blood  sinuses.  The  wall  of  the  cylinder  is  the 
outer  wall  of  the  body  itself,  to  which  the  general  name 
of  integument  may  be  given  ;  and  the  outermost  layer  of 
this,  again,  is  the  cuticle,  which  gives  rise  to  the  whole 
of  the  exoskeleton.  This  cuticle,  as  we  have  seen,  is 
extensively  impregnated  with  lime  salts ;  and,  moreover, 
in  consequence  of  its  containing  chitin,  it  is  often  spoken 
of  as  the  chitinous  cuticula. 

Having  arrived  at  this  general  conception  of  the  dis- 
position of  the  parts  of 'the  factory,  we  m;iy  next  proceed 
to  consider  the  machinery  of  alimentation  which  is  con- 


THE   MACHINERY  OF  ALIMENTATION.  51 

tained  within  it,  and  which  is  represented  by  the  various 
divisions  of  the  alimentary  canal,  with  its  appendages ; 
by  the  apparatus  for  the  distribution  of  nutriment ;  and 
by  two  apparatuses  for  getting  rid  of  those  products 
which  are  the  ultimate  result  of  the  working  of  the  whole 
organism. 

And  here  we  must  trench  somewhat  upon  the  province 
of  Morphology,  as  some  of  these  pieces  of  apparatus  are 
complicated ;  and  their  action  cannot  be  comprehended 
without  a  certain  knowledge  of  their  anatomy. 

The  mouth  of  the  crayfish  is  a  longitudinally  elongated, 
parallel-sided  opening,  in  the  integument  of  the  ventral 
or  sternal  aspect  of  the  head.  Just  outside  its  lateral 
boundaries,  the  strong  mandibles  project,  one  on  each 
side  (fig  3,  B  ;  4) ;  their  broad  crushing  surfaces,  which 
are  turned  towards  one  another,  are  therefore  completely 
external  to  the  oral  cavit}r.  In  front,  the  mouth  is  over- 
lapped by  a  wide  shield-shaped  plate  termed  the  upper 
lip,  or  labrum  (figs.  3  and  6,  Ib) ;  while,  immediately  be- 
hind the  mandibles,  there  is,  on  each  side,  an  elongated 
fleshy  lobe,  joined  with  its  fellow  by  the  posterior 
boundary  of  the  mouth.  These  together  constitute  the 
rnetastoma  (fig.  3,  B ;  mt),  which  is  sometimes  called 
the  lower  lip.  A  short  wide  gullet,  termed  the  oeso- 
phagus (fig.  6,  oe),  leads  directly  upwards  into  a  spacious 
bag,  the  stomach,  which  occupies  almost  the  whole  cavity 
of  the  head.  It  is  divided  by  a  constriction  into  a  large 
anterior  chamber  (cs),  into  the  under  face  of  which  the 


52    THE  PHYSIOLOGY  OF  THE  COMMON  CRAYFISH. 

gullet  opens,  and  a  small  posterior  chamber  (ps),  from 
which  the  intestine  (hg)  proceeds. 

In  a  man's  stomach,  the  opening  by  which  the  gullet 
communicates  with  the  stomach  is  called  the  cardia, 
while  that  which  places  the  stomach  in  communication 
with  the  intestine  is  named  the  pylorus ;  and  these  terms 
having  been  transferred  from  human  anatomy  to  that  of 
the  lower  animals,  the  larger  moiety  of  the  crayfish's 
stomach  is  called  the  cardiac  division,  while  the  smaller 
is  termed  the  pyloric  division  of  the  organ.  It  must  be 
recollected,  however,  that,  in  the  crayfish,  the  so-called 
cardiac  division  is  that  which  is  actually  furthest  from 
the  heart,  not  that  which  is  nearest  to  it,  as  in  man. 

The  gullet  is  lined  by  a  firm  coat  which  resembles  thin 
parchment.  At  the  margins  of  the  mouth,  this  strong 
lining  is  easily  seen  to  be  continuous  with  the  cuticular 
exoskeleton ;  while,  at  the  cardiac  orifice,  it  spreads  out 
and  forms  the  inner  or  cuticular  wall  of  the  whole  gastric 
cavity,  as  far  as  the  pylorus,  where  it  ends  in  certain 
valvular  projections.  The  chitinous  cuticle  which  forms 
the  outermost  layer  of  the  integument  is  thus,  as  it  were, 
turned  in,  to  constitute  the  innermost  layer  of  the  walls 
of  the  stomach;  and  it  confers  upon  them  so  great  an 
amount  of  stiffness  that  they  do  not  collapse  when  the 
organ  is  removed  from  the  bodj7.  Furthermore,  just  as 
the  cuticle  of  the  integument  is  calcified  to  form  the  hard 
parts  of  the  exoskeleton,  so  is  the  cuticle  of  the  stomach 
calcified,  or  otherwise  hardened,  to  give  rise,  in  the  first 


THE  STOMACH  OF  THE  CRAYFISH. 


53 


place,  to  the  very  remarkable  and  complicated  apparatus 
which  has  already  been  spoken  of,  as  a  sort  of  gastric  mill 


P.c 


Fi3.  9.— Astacus  fluriatilis.—  A,  the  stomach  with  its  outer  coat  removed,  seen  from  the 
left  side  ;  B,  the  same  viewed  from  the  front,  after  removal  of  the  anterior  wall ; 
C,  the  ossicles  of  the  gastric  mill  separated  from  one  another ;  D,  the  prepy- 
lorie  ossicle  and  median  tooth,  seen  from  the  right  side ;  E,  transverse  section  of 
the  pyloric  region  along  the  line  xy  in  A  (all  x  2).  c,  cardiac  ossicle  ;  cpv,  cardio- 
pylorie  valve  ;  lp,  lateral  pouch  :  It,  lateral  tooth,  seen  through  the  wall  of  the 
stomach  in  A  ;  ing,  mid-gut ;  mt,  median  tooth,  seen  through  the  wall  of  the 
stomach  in  A  ;  ces,  oesophagus ;  p,  pyloric  ossicle ;  pc,  pterocardiac  ossicle  ; 
pp,  prepyloric  ossicle  ;  vc,  uro-cardiac  process  ;  t,  convexities  on  the  free  surface 
of  its  hinder  end  ;  vl,  median  pyloric  valve  ;  zc,  zygocardiac  ossicle. 

or  food-crusher ;  and,  secondly,  to  a  filter  or  strainer, 
whereby  the  nutritive  juices  are  separated  from  the  in- 
nutritious  hard  parts  of  the  food  and  passed  on  into  the 
intestine. 


54         THE  PHYSIOLOGY  OF   THE   COMMON   CRAYFISH. 

The  gastric  mill  begins  in  the  hinder  half  of  the  cardiac 
division.  Here,  on  the  upper  wall  of  the  stomach,  we  see 
a  broad  transverse  calcined  bar  (figs.  9-11,  c)  from 
the  middle  of  the  hinder  part  of  which  another  bar  (we), 
united  to  the  first  by  a  flexible  portion,  is  continued 
backwards  in  the  middle  line.  The  whole  has,  therefore, 
somewhat  the  shape  of  a  cross-bow.  Behind  the  first- 
mentioned  piece,  the  dorsal  wall  of  the  stomach  is  folded 
in,  in  such  a  manner  as  to  give  rise  to  a  kind  of  pouch ; 
and  the  second  piece,  or  what  we  may  call  the  handle  of 
the  crossbow,  lies  in  the  front  wrall  of  this  pouch.  The 
end  of  this  piece  is  dense  and  hard,  and  its  free  surface, 
which  looks  into  the  top  of  the  cardiac  chamber,  is 
raised  into  two  oval,  flattened  convex  surfaces  (£).  Con- 
nected by  a  transverse  joint  with  the  end  of  the  handle 
of  the  crossbow,  there  is  another  solid  bar,  which  ascends 
obliquely  forwards  in  the  back  wall  of  the  pouch  (pp). 
The  end  which  is  articulated  with  the  handle  of  the  cross- 
bow is  produced  into  a  strong  reddish  conical  tooth  (mt), 
curved  forwards  and  bifurcated  at  the  summit ;  conse- 
quently, when  the  cavity  of  the  stomach  is  inspected  from 
the  fore  part  of  the  cardiac  pouch  (fig.  9,  B),  the  two- 
pointed  curved  tooth  (mt)  is  seen  projecting  behind  the 
convex  surfaces  (t),  in  the  middle  line,  into  the  interior 
of  that  cavity.  The  joint  which  connects  the  handle  of  the 
crossbow  with  the  hinder  middle  piece  is  elastic  ;  hence, 
if  the  two  are  straightened  out,  they  return  to  their  bent  dis- 
position as  soon  as  they  are  released.  The  upper  end  of 


THE    GASTKIC    MILL.  55 

the  hinder  middle  piece  (pp)  is  connected  with  a  second  flat 
transverse  plate  which  lies  in  the  dorsal  wall  of  the  pyloric 
chamber  (p).  The  whole  arrangement,  thus  far,  may  be 
therefore  compared  to  a  large  cross-bow  and  a  small  one, 
with  the  ends  of  their  handles  fastened  together  by  a 
spring  joint,  in  such  a  manner  that  the  handle  of  the 
one  makes  an  acute  angle  with  the  handle  of  the  other ; 
while  the  middle  of  each  bow  is  united  with  the  middle  of 
the  other  by  the  bent  arm  formed  by  the  two  handles. 
But,  in  addition  to  this,  the  outer  ends  of  the  two  bows 
are  also  connected  together.  A  small,  curved,  calcined 
bar  (pc)  passes  from  the  outer  end  of  the  front  crosspiece 
downwards  and  outwards  in  the  wall  of  the  stomach,  and 
its  hinder  and  lower  extremity  is  articulated  with  another 
larger  bar  (zc)  which  runs  upwards  and  backwards  to 
the  hinder  or  pyloric  crosspiece,  with  which  it  articulates. 
Internally,  this  piece  projects  into  the  cardiac  cavity  of 
the  stomach  as  a  stout  elongated  reddish  elevation  (It), 
the  surface  of  which  is  produced  into  a  row  of  strong 
sharp,  transverse  ridges,  which  diminish  in  size  from 
before  backwards,  and  constitute  a  crushing  surface 
almost  like  that  of  the  grinder  of  an  elephant.  Thus, 
when  the  front  part  of  the  cardiac  cavity  is  cut  away, 
not  only  are  the  median  teeth  already  mentioned  seen, 
but,  on  each  side  of  them,  there  is  one  of  these  long 
lateral  teeth. 

There  are  two  small  pointed  teeth,  one   under  each 
of  the  lateral  teeth,  and  each  of  these  is  supported  bj 


56 


THE   PHYSIOLOGY   OF   THE   COMMON   CRAYFISH. 


a  broad  plate,  hairy  on  its  inner  surface,  which  enters 
into  the  lateral  wall  of  the  cardiac  chamber.  There  are 
various  other  smaller  skeletal  parts,  but  the  most  im- 


FIG.  10. — Astacusfluviatilis. — Longitudinal  section  of  the  stomach  ( x  4), 
c,  cardiac  ossicle;  C(P,  caecum  ;  c.p.v,  cardio-pyloric  valve ;  cs.  cushion- 
shaped  surface  ;  //</,  hind-gut  ;  lip,  aperture  of  right  bile  duct  ;  lp, 
lateral  pouch  ;  It,  lateral  teeth  ;  nig,  mid-gut ;  mt,  median  tooth  ;  «-*, 
oesophagus  ;  p,  pyloric  ossicle  ;  pc.  pterocardiac  ossicles  ;  pp,  prepy- 
loric  ossicle  ;  iic,  urocardiac  process  ;  rl,  median  pyloric  valve;  r-, 
lateral  pyloric  valve;  a?,  position  of  gastrolith;  zc,  zygocardiac  ossicle. 

portant  are  those  which  have  been  described ;  and  these, 
from  what  has  been  said,  will  be  seen  to  form  a  sort  of 
hexagonal  frame,  with  more  or  less  flexible  joints  at  the 
angles,  and  having  the  anterior  and  the  posterior  sides 


THE    GASTRIC    MILL,  5? 

connected  by  a  bent  jointed  middle  bar.  As  all  these 
parts  are  merely  modifications  of  the  hard  skeleton,  the 
apparatus  is  devoid  of  any  power  of  moving  itself.  It 
is  set  in  motion,  however,  by  the  same  substance  as  that 
which  gives  rise  to  all  the  other  bodily  movements  of 
the  crayfish,  namely,  muscle.  The  chief  muscles  which 
move  it  are  four  very  strong  bundles  of  fibres.  Two  01 
these  are  attached  to  the  front  crosspiece,  and  proceed 
thence,  upwards  and  forwards,  to  be  fixed  to  the  inner  face 
of  the  carapace  in  the  front  part  of  the  head  (figs.  5,  6, 
and  12,  ag).  The  two  others,  which  are  fixed  into  the 
hinder  crosspiece  and  hinder  lateral  pieces,  pass  upwards 
and  backwards,  to  be  attached  to  the  inner  face  of  the 
carapace  in  the  back  part  of  the  head  (pg).  When  these 
muscles  shorten,  or  contract,  they  pull  the  front  and  back 
crosspieces  further  away  from  one  another ;  consequently, 
the  angle  between  the  handles  becomes  more  open  and 
the  tooth  which  is  borne  on  their  ends  travels  downwards 
and  forwards.  But,  at  the  same  time,  the  angle  between 
the  side  bars  becomes  more  open  and  the  lateral  tooth 
of  each  side  moves  inwards  till  it  crosses  in  front  of  the 
middle  tooth,  and  strides  against  this  and  the  opposite 
lateral  tooth,  which  has  undergone  a  corresponding  change 
of  place.  The  muscles  being  now  relaxed,  the  elasticity 
of  the  joints  suffices  to  bring  the  whole  apparatus  back 
to  its  first  position,  when  a  new  contraction  brings  about 
a  new  clashing  of  the  teeth.  Thus,  by  the  alternate  con- 
traction and  relaxation  of  these  two  pair  of  muscles,  the 


58         THE   PHYSIOLOGY   OF   THE   COMMON    CRAYFISH. 

three  teeth  are  made  to  stir  up  and  crush  whatever  ia 
contained  in  the  cardiac  chamber.  When  the  stomach  is 
removed  and  the  front <part  of  the  cardiac  chamber  is  cut 
away,  the  front  cross-piece  may  be  seized  with  one  pair 
of  forceps  and  the  hind  cross-piece  with  another.  On 
slightly  pulling  the  two,  so  as  to  imitate  the  action  of  the 
muscles,  the  three  teeth  will  be  found  to  corne  together 
sharply,  exactly  in  the  manner  described. 

Works  on  mechanics  are  full  of  contrivances  for  the 
conversion  of  motion;  but  it  would,  perhaps,  be  difficult  to 
discover  among  these  a  prettier  solution  of  the  problem  ; 
given  a  straight  pull,  how  to  convert  it  into  three  simul- 
taneous convergent  movements  of  as  many  points. 

What  I  have  called  the  filter  is  constructed  mainly  out 
of  the  chitinous  lining  of  the  pyloric  chamber.  The  aper- 
ture of  communication  between  this  and  the  cardiac 
chamber,  already  narrow,  on  account  of  the  constriction  of 
the  walls  of  the  stomach  at  this  point,  is  bounded  at  the 
sides  by  two  folds ;  while,  from  below,  a  conical  tongue- 
shaped  process  (figs.  6,  10,  and  11,  cpv),  the  surface  of 
which  is  covered  with  hairs,  further  obstructs  the  opening. 
In  the  posterior  half  of  the  pyloric  chamber,  its  side  walls 
are,  as  it  were,  pushed  in;  and,  above,  they  so  nearly  meet 
in  the  middle  line,  that  a  mere  vertical  chink  is  left  be- 
tween them ;  while  even  this  is  crossed  by  hairs  set  upon 
the  two  surfaces.  In  its  lower  half,  however,  each  side 
wall  curves  outwards,  and  forms  a  cushion-shaped  surface 
(fig.  10,  cs)  which  looks  downwards  and  inwards.  If  the 


THE  FILTERING  APPARATUS.  59 

floor  of  the  pyloric  chamber  were  flat,  a  wide  triangular 
passage  would  thus  be  left  open  in  its  lower  half.  But, 
in  fact,  the  floor  rises  into  a  ridge  in  the  middle,  while,  at 
the  sides,  it  adapts  itself  to  the  shape  of  the  two  cushion- 
shaped  surfaces ;  the  result  of  which  is  that  the  whole 
cavity  of  the  posterior  part  of  the  pyloric  division  of  the 
stomach  is  reduced  to  a  narrow  three-rayed  fissure.  In 
transverse  section,  the  vertical  ray  of  this  fissure  is 
straight,  while  the  two  lateral  ones  are  concave  upwards 
(fig.  9,  E).  The  cushions  of  the  side  walls  are  covered 
with  short  close-set  hairs.  The  corresponding  surfaces 
of  the  floor  are  raised  into  longitudinal  parallel  ridges, 
the  edge  of  each  of  which  is  fringed  with  very  fine  hairs. 
As  everything  which  passes  from  the  cardiac  sac  to  the 
intestine  must  traverse  this  singular  apparatus,  only  the 
most  finely  divided  solid  matters  can  escape  stoppage,  so 
long  as  its  walls  are  kept  together. 

Finally,  at  the  opening  of  the  pyloric  sac  into  the 
intestine,  the  chitinous  investment  terminates  in  five 
symmetrically  arranged  processes,  the  disposition  of 
which  is  such  that  they  must  play  the  part  of  valves 
in  preventing  any  sudden  return  of  the  contents  of  the 
intestine  to  the  stomach,  while  they  readily  allow  of  a 
passage  the  other  way.  One  of  these  valvular  processes 
is  placed  in  the  middle  line  above  (figs.  10  and  11,  r1). 
It  is  longer  than  the  others  and  concave  below.  The 
lateral  processes  (z;2,)  of  which  there  are  two  on  each  side, 
are  triangular  and  flat. 


60 


THE  PHYSIOLOGY  OF  THE  COMMON  CRAYFISH. 


The  cuticular  lining  which  gives  rise  to  all  the  com- 
plicated apparatus  which  has  just  been  described,  must 


771 .9 


FIG.  11. — Astacus  fluviatilu.— View  of  the  roof  of  the  stomach,  the 
ventral  wall  of  which,  and  of  the  mid-gut,  is  laid  open  by  a  longi- 
tudinal incision  (  x  4).  On  the  right  side  (the  left  in  the  figure), 
the  lateral  tooth  is  cut  away,  as  well  as  the  floor  of  the  lateral 
pouch.  The  letters  have  the  same  signification  as  in  fig.  10. 

not  be  confounded  with  the  proper  wall  of  the  stomach, 
which  invests  it,  and  to  which  it  owes  it  origin,  just  as 
the  cuticle  of  the  integument  is  produced  by  the  soft 


FORE-GUT,  MID-GUT,  AND  HIND-GUT.  61 

tnie  skin  which  lies  heneath  it.  The  wall  of  the  stomach 
is  a  soft  pale  membrane  containing  variously  disposed 
muscular  fibres  ;  and,  beyond  the  pylorus,  it  is  continued 
into  the  wall  of  the  intestine. 

It  has  already  been  mentioned  that  the  intestine  is 
a  slender  and  thin-walled  tube,  which  passes  straight 
through  the  body  almost  without  change,  except  that  it 
becomes  a  little  wider  and  thicker-walled  near  the  vent. 
Immediately  behind  the  pyloric  valves,  its  surface  is  quite 
smooth  and  soft  (figs.  9,  10,  and  12,  mg),  and  its  floor 
presents  a  relatively  large  aperture,  the  termination  of 
the  bile  duct  (fig.  12,  bd,  fig.  10,  hp.),  on  each  side.  The 
roof  is,  as  it  were,  pushed  out  into  a  short  median  pouch 
or  ccecum  (cce).  Behind  this,  its  character  suddenly 
changes,  and  six  squarish  elevations,  covered  with  a 
chitinous  cuticle,  encircle  the  cavity  of  the  intestine  (r). 
From  each  of  these,  a  longitudinal  ridge,  corresponding 
with  a  fold  of  the  wall  of  the  intestine,  takes  its  rise,  and 
passes,  with  a  slight  spiral  twist,  to  its  extremity  (hg). 
Each  of  these  ridges  is  beset  with  small  papillae,  and  the 
chitinous  lining  is  continued  over  the  whole  to  the  vent, 
where  it  passes  into  the  general  cuticle  of  the  integu- 
ment, just  as  the  lining  of  the  stomach  is  continuous 
with  the  cuticle  of  the  integument  at  the  mouth.  The 
alimentary  canal  may,  therefore,  be  distinguished  into 
a  fore  and  a  hind-gut  (hg),  which  have  a  thick  internal 
lining  of  cuticular  membrane  ;  and  a  very  short  mid- 
gut  (mg),  which  has  no  thick  cuticular  layer.  It  will  be  of 


THE   DIGESTION   OF   FOOD.  63 

importance  to  recollect  this  distinction  by-and-by,  when 
the  development  of  the  alimentary  canal  is  considered. 

If  the  treatment  to  which  the  food  is  subjected  in 
the  alimentary  apparatus  were  of  a  purely  mechanical 
nature,  there  would  be  nothing  more  to  describe  in  this 
part  of  the  crayfish's  mechanism.  But,  in  order  that 
the  nutritive  matters  may  be  turned  to  account,  and 
undergo  the  chemical  metamorphoses,  which  eventually 
change  them  into  substances  of  a  totally  different  cha- 
racter, they  must  pass  out  of  the  alimentary  canal  into 
the  blood.  And  they  can  do  this  only  by  making  their 
way  through  the  walls  of  the  alimentary  canal ;  to  which 
end  they  must  either  be  in  a  state  of  extremely  fine 
division,  or  they  must  be  reduced  to  the  fluid  condition. 
In  the  case  of  the  fatty  matters,  minute  subdivision  may 
suffice ;  but  the  amylaceous  substances  and  the  insoluble 
protein  compounds,  such  as  the  fibrin  of  flesh,  must  be 
brought  into  a  state  of  solution.  Therefore  some  sub- 
stances must  be  poured  into  the  alimentary  canal,  which, 
when  mixed  with  the  crushed  food,  will  play  the  part 
of  a  chemical  agent,  dissolving  out  the  insoluble  proteids, 
changing  the  amyloids  into  soluble  sugar,  and  convert- 
ing all  the  proteids  into  those  diffusible  forms  of  protein 
matter,  which  are  known  as  peptones. 

The  details  of  the  processes  here  indicated,  which 
may  be  included  under  the  general  name  of  digestion,  have 
only  quite  recently  been  carefully  investigated  in  the 
crayfish ;  and  we  have  probably  still  much  to  learn  about 


64        THE  PHYSIOLOGY   OF  THE   COMMON   CRAYFISH. 

them;  but  what  has  heen  made  out  is  very  interesting, 
and  proves  that  considerable  differences  exist  between 
crayfishes  and  the  higher  animals  in  this  respect. 

The  physiologist  calls  those  organs,  the  function  of 
which  is  to  prepare  and  discharge  substances  of  a  special 
character,  glands;  and  the  matter  which  they  elaborate 
is  termed  their  secretion.  On  the  one  side,  glands  are 
in  relation  with  the  blood,  whence  they  derive  the 
materials  which  they  convert  into  the  substances 
characteristic  of  their  secretion ;  on  the  other  side, 
they  have  access,  directly  or  indirectly,  to  a  free  surface, 
on  to  which  they  pour  their  secretion  as  it  is  formed. 

Of  such  glands,  the  alimentary  canal  of  the  crayfish 
is  provided  with  a  pair,  which  are  not  only  of  very  large 
size,  but  are  further  extremely  conspicuous,  on  account 
of  their  yellow  or  brown  colour.  These  two  glands  (figs. 
12  and  13,  IT)  are  situated  beneath,  and  on  each  side  of,  the 
stomach  and  the  anterior  part  of  the  intestine,  and  answer 
in  position  to  the  glands  termed  liver  and  pancreas  in  the 
higher  animals,  inasmuch  as  they  pour  their  secretion  into 
the  mid-gut.  These  glands  have  hitherto  always  been  re- 
garded as  the  liver,  and  the  name  may  be  retained,  though 
their  secretion  appears  rather  to  correspond  with  the 
pancreatic  fluid  than  with  the  bile  of  the  higher  animals. 

Each  liver  consists  of  an  immense  number  of  short 
tubes,  or  caca,  which  are  closed  at  one  end,  but  open  at 
the  other  into  a  general  conduit,  which  is  termed  their 
duct.  The  mass  of  the  liver  is  roughly  divided  into 


FIG.  IS.—Agtacusfluviattti*.  -The  alimentary  canal  and  livers  seen  from 
above  (nat.  size),  bd.  bile-duct ;  cte,  caecum  ;  cs,  cardiac  portion  of 
stomach,  the  line  pointing  to  the  cardiac  ossicle  ;  /iff,  hind- grit ;  mg, 
mid-gut ;  pc,  pterocardiac  ossicle  ;  ps,  pyloric  portion  of  stomach, 
the  line  pointing  to  the  pyloric  ossicle  ;  r,  ridge  separating  mid-gut 
from  hind-gut ;  zc,  zygocardiac  ossicle. 
6 


66    THE  PHYSIOLOGY  OF  THE  COMMON  CRAYFISH. 

three  lobes,  one  anterior,  one  lateral,  and  one  posterior  ; 
and  each  lobe  has  its  main  duct,  into  which  all  the  tubes 
composing  it  open.  The  three  ducts  unite  together  into 
a  wide  common  duct  (bd),  which  opens,  just  behind  the  py- 
loric  valves,  into  the  floor  of  the  mid-gut.  Hence  the  aper- 
tures of  the  two  hepatic  ducts  are  seen,  one  on  each  side, 
in  this  part  of  the  alimentary  canal  when  it  is  laid  open 
from  above.  Every  caecum  of  the  liver  has  a  thin  outer 
wall,  lined  internally  by  a  layer  of  cells,  constituting  what 
is  termed  an  epithelium ,-  and,  at  the  openings  of  the 
hepatic  ducts,  this  epithelium  passes  into  a  layer  of  some- 
what similar  structure,  which  lines  the  mid-gut,  and  is 
continued  through  the  rest  of  the  alimentary  canal, 
beneath  the  cuticula.  Hence  the  liver  may  be  regarded 
as  a  much  divided  side  pouch  of  the  mid-gut. 

The  epithelium  is  made  up  of  nucleated  cells,  which  are 
particles  of  simple  living  matter,  or  protoplasm,  in  the 
midst  of  each  of  which  is  a  rounded  body,  which  is  termed 
the  nucleus.  It  is  these  cells  which  are  the  seat  of  the 
manufacturing  process  which  results  in  the  formation  of 
the  secretion ;  it  is,  as  it  were,  their  special  business  to 
form  that  secretion.  To  this  end  they  are  constantly  being 
newly  formed  at  the  summits  of  the  caeca.  As  they  grow, 
they  pass  down  towards  the  duct  and,  at  the  same  time, 
separate  into  their  interior  certain  special  products, 
among  which  globules  of  yellow  fatty  matter  are  very 
conspicuous.  When  these  products  are  fully  formed,  what 
remains  of  the  substance  of  the  cells  dissolves  away,  and 


THE  DIGESTION   OF  FOOD.  67 

the  yellow  fluid  accumulating  in  the  ducts  passes  into 
the  mid- gut.  The  yellow  colour  is  due  to  the  globules  of 
fat.  In  the  young  cells,  at  the  summit  of  the  caeca, 
these  are  either  absent,  or  very  small,  whence  the  part 
appears  colourless.  But,  lower  down,  small  yellow 
granules  appear  in  the  cells,  and  these  become  bigger 
and  more  numerous  in  the  middle  and  lower  pails.  In 
fact,  few  glands  are  better  fitted  for  the  study  of  the 
manner  in  which  secretion  is  effected  than  the  crayfish's 
liver. 

We  may  now  consider  the  alimentary  machinery,  the 
general  structure  of  which  has  been  explained,  in 
action. 

The  food,  already  torn  and  crushed  by  the  jaws,  is 
passed  through  the  gullet  into  the  cardiac  sac,  and  there 
reduced  to  a  still  more  pulpy  state  by  the  gastric  mill. 
By  degrees,  such  parts  as  are  sufficiently  fluid  are 
drained  off  into  the  intestine,  through  the  pyloric  strainer, 
while  the  coarser  pails  of  the  useless  matters  are  probably 
rejected  by  the  mouth,  as  a  hawk  or  an  owl  rejects  his 
casts.  There  is  reason  to  believe,  though  it  is  not  certainly 
known,  that  fluids  from  the  intestine  mix  with  the  food 
while  it  is  undergoing  trituration,  and  effect  the  transforma- 
tion of  the  starchy  and  the  insoluble  protein  compounds 
into  a  soluble  state.  At  any  rate,  as  soon  as  the  strained -off 
fluid  passes  into  the  mid-gut  it  must  be  mixed  with  the 
secretion  of  the  liver,  the  action  of  which  is  probably 


68 


THE   PHYSIOLOGY   OF   THE   COMMON    CRAYFISH. 


similar  to  that  of  the  pancreatic   juice    of  the    higher 
animals. 

The  mixture  thus  produced,  which  answers  to  the 
chyle  of  the  higher  animals,  passes  along  the  intestine, 
and  the  greater  part  of  it,  transuding  through  the  walls  of 
the  alimentary  canal,  enters  the  blood,  while  the  rest 
accumulates  as  dark  coloured  faeces  in  the  hind  gut,  and 


&1&*l 


FIG.  14. — Astacm  jturiatilis.— The  corpuscles  of  the  blood  (highly  mag- 
nified). 1-8  show  the  changes  undergone  by  a  single  corpuscle 
during  a  quarter  of  an  hour  ;  0  and  10  are  corpuscles  killed  by 
magenta,  and  having  the  nucleus  deeply  stained  by  the  colouring 
matter,  n,  nucleus. 


is  eventually  passed  out  of  the  body  by  the  vent.  The 
faBcal  matters  are  small  in  amount,  and  the  strainer  is 
so  efficient  that  they  rarely  contain  solid  particles  of 
sensible  size.  Sometimes,  however,  there  are  a  good 
many  minute  fragments  of  vegetable  tissue. 

The  blood  of  which  the  nutritive  elements  of  the  food 


THE  BLOOL.  AND    ITS  CORPUSCLES.  69 

have  thus  become  integral  parts,  is  a  clear  fluid,  either 
colourless,  or  of  a  pale  neutral  tint  or  reddish  hue,  which, 
to  the  naked  eye,  appears  like  so  much  water.  But  if 
subjected  to  microscopic  examination,  it  is  found  to  con- 
tain innumerable  pale,  solid  particles,  or  corpuscles, 
which,  when  examined  fresh,  undergo  constant  changes 
of  form  (fig.  14).  In  fact,  they  correspond  very  closely 
with  the  colourless  corpuscles  which  exist  in  our  own 
blood;  and,  in  its  general  characters,  the  crayfish's 
blood  is  such  as  ours  would  be  if  it  were  somewhat 
diluted  and  deprived  of  its  red  corpuscles.  In  other 
words,  it  resembles  our  lymph  more  than  it  does  our 
blood.  Left  to  itself  it  soon  coagulates,  giving  rise  to  a 
pretty  firm  clot. 

The  sinuses,  or  cavities  in  which  the  greater  part  of 
the  blood  is  contained,  are  disposed  very  irregularly  in 
the  intervals  between  the  internal  organs.  But  there  is 
one  of  especially  large  size  on  the  ventral  or  sternal  side 
of  the  thorax  (fig.  15,  sc),  into  which  all  the  blood  in  the 
body  sooner  or  later  makes  its  way.  From  this  sternal 
sinus  passages  (av)  lead  to  the  gills,  and  from  these  again 
six  canals  (bcv),  pass  up  on  the  inner,  side  of  the  inner  wall 
of  each  branchial  chamber  to  a  cavity  situated  in  the 
dorsal  region  of  the  thorax,  termed  the  pericardium  (p), 
into  which  they  open. 

The  blood  of  the  crayfish  is  kept  in  a  state  of  con- 
stant circulating  motion  by  a  pumping  and  distributing 
machinery,  composed  of  the  heart  and  of  the  arteriest  with 


fp- 


gn.6. 


FlG.  15. — AstacH*  Jiuvlatilis.—A  diagrammatic  transverse  section  of 
the  thorax  through  the  twelfth  somite,  showing  the  course  of  the 
circulation  of  the  blood  (  x  3).  arb.  12,  the  anterior  or  lower,  and 
arV.  12,  the  posterior  or  upper  arthrobranchia  of  the  twelfth 
somite  ;  av,  afferent  branchial  vessel  ;  bcr,  branchio-cardiac  vein  ; 
bff,  branchiostegite ;  em.,  extensor  muscles  of  abdomen  ;  fp,  epi- 
meral  wall  of  thoracic  cavity  ;  er.  efferent  branchial  vessel ;  fm, 
flexor  muscles  of  abdomen  ;  fp,  floor  of  pericardium  ;  gn.  6,  fifth 
thoracic  ganglion  ;  h.  heart  ;  Jig,  hind-gut ;  ma,  inferior  abdominal 
artery,  in  cross  section  ;  la,  lateral  valvular  apertures  of  heart  :  Ir, 
liver  ;  mp,  indicates  the  position  of  the  mesophragm  by  which  the 
sternal  canal  is  bounded  laterally  ;  p.  pericardial  sinus  ;  pdb.  12, 
podobranchia,  and  plb.  12,  pleurobranchia  of  the  twelfth  somite  ; 
m,sternal  artery ;  saa,  superior  abdominal  artery  ;  sc,  sternal  canal ; 
t,  testis  ;  xii.,  sternum  of  twelfth  somite.  The  arrows  indicate 
the  direction  of  the  blood  flow. 


THE  HEART  AND   THE  ARTERIES.  71 

their  larger  and  smaller  branches,  which  proceed  from  it 
and  ramify  through  the  body,  to  terminate  eventually  in 
the  blood  sinuses,  which  represent  the  veins  of  the 
higher  animals. 

When  the  carapace  is  removed  from  the  middle  of  the 
region  which  lies  behind  the  cervical  groove,  that  is, 
when  the  dorsal  or  tergal  wall  of  the  thorax  is  taken 
away,  a  spacious  chamber  is  laid  open  which  is  full  of 
blood.  This  is  the  cavity  already  mentioned  as  the  peri- 
cardium (fig.  15,  p),  though,  as  it  differs  in  some  respects 
from  that  which  is  so  named  in  the  higher  animals,  it  will 
be  better  to  term  it  the  pericardia!  sinus. 

The  heart  (fig.  15,  /i),  lies  in  the  midst  of  this  sinus.  It 
is  a  thick  muscular  body  (fig.  16),  with  an  irregularly  hexa- 
gonal contour  when  viewed  from  above,  one  angle  of  the 
hexagon  being  anterior  and  another  posterior.  The  lateral 
angles  of  the  hexagon  are  connected  by  bands  of  fibrous  tis- 
sue (ac)  with  the  walls  of  the  pericardial  sinus.  Otherwise, 
the  heart  is  free,  except  in  so  far  as  it  is  kept  in  place  by  the 
arteries  which  leave  it  and  traverse  the  walls  of  the  peri- 
cardium. One  of  these  arteries  (figs.  5,  12,  and  16,  saa), 
starting  from  the  hinder  part  of  the  heart,  of  which  it 
is  a  sort  of  continuation,  runs  along  the  middle  line  of 
the  abdomen  above  the  intestine,  to  which  it  gives  off 
many  branches.  A  second  large  artery  starts  from  a 
dilatation,  which  is  common  to  it  with  the  foregoing,  but 
passing  directly  downwards  (figs.  12  and  15,  sa,  and  fig.  16, 
9t.  a),  either  on  the  right  or  on  the  left  side  of  the  intestine, 


72        THE   PHYSIOLOGY   OF   THE  COMMON    CRAYFISH. 


traverses  the  nervous  cord  (figs.  12  and  15),  and  divides 
into  an  anterior  (fig.  12,  sa)  and  a  posterior  (iaa)  branch, 
both  of  which  run  beneath  and  parallel  with  that  cord. 


Mb 

rfSf/ 


0.0-. 


fir. 


st.a> 


FIG.  16.— Astacmfluvlatilis.—  The  heart  (  x  4).  A,  from  above  ;  B,  from 
below  ;  C,  from  the  left  side,  aa,  antennary  artery  ;  ac,  alee  cordis, 
or  fibrous  bands  connecting  the  heart  with  the  walls  of  the  peri- 
cardial  sinus  ;  #,  bulbous  dilatation  at  the  origin  of  the  sternal 
artery  ;  ha,  hepatic  artery  ;  la,  lateral  valvular  apertures  ;  oa,  oph- 
thalmic artery  ;  s.a,  superior  valvular  apertures  ;  x.a.a,  superior 
abdominal  artery  ;  st.a,  sternal  artery,  in  B  cut  off  close  to  its 
origin. 

A  third  artery  runs,  from  the  front  part  of  the  heart, 
forwards  in  the  middle  line,  over  the  stomach,  to  the 
eyes  and  fore  part  of  the  head  (figs.  5,  12,  and  16,  oa)  ; 
and  two  others  diverge  one  on  each  side  of  this,  and  sweep 


THE  ACTION  OF   THE  HEART.  73 

round  the  stomach  to  the  antennae  (aa).  Behind  these, 
yet  two  other  arteries  are  given  off  from  the  under  side  of 
the  heart,  and  supply  the  liver  (ha).  All  these  arteries 
branch  out  and  eventually  terminate  in  fine,  so-called 
capillary,  ramifications. 

In  the  dorsal  wall  of  the  heart  two  small  oval  aper* 
tures  are  visible,  provided  with  valvular  lips  (fig.  16, 
sa),  which  open  inwards,  or  towards  the  internal  cavity 
of  the  heart.  There  is  a  similar  aperture  in  each  of  the 
lateral  faces  of  the  heart  (la),  and  two  others  in  its 
inferior  face  (id),  making  six  in  all.  These  apertures 
readily  admit  fluid  into  the  heart,  but  oppose  its  exit. 
On  the  other  hand,  at  the  origins  of  the  arteries,  there 
are  small  valvular  folds,  directed  in  such  a  manner  as  to 
permit  the  exit  of  fluid  from  the  heart,  while  they  prevent 
its  entrance. 

The  walls  of  the  heart  are  muscular,  and,  during  life, 
they  contract  at  intervals  with  a  regular  rhythm,  in  such 
a  manner  as  to  diminish  the  capacity  of  the  internal  cavity 
of  the  organ.  The  result  is,  that  the  blood  which  it 
contains  is  driven  into  the  arteries,  and  necessarily  forces 
into  their  smaller  ramifications  an  equivalent  amount  of 
the  blood  which  they  already  contained ;  whence,  in  the 
long  run,  the  same  amount  of  blood  passes  out  of  the 
ultimate  capillaries  into  the  blood  sinuses.  From  the 
disposition  of  the  blood  sinuses,  the  impulse  thus  given 
to  the  blood  which  they  contain  is  finally  conveyed  to  the 
blood  in  the  branchiae,  and  a  proportional  quantity  of  that 


74        THE  PHYSIOLOGY  OF  THE  COMMON   CRAYFISH. 

blood  leaves  the  branchiae  and  passes  into  the  sinuses  which 
connect  them  with  the  pericardial  sinus  (fig.  15,  lev),  and 
thence  into  that  cavity.  At  the  end  of  the  contraction, 
or  systole,  of  the  heart,  its  volume  is  of  course  diminished 
by  the  volume  of  the  blood  forced  out,  and  the  space 
between  the  walls  of  the  heart  and  those  of  the  pericardial 
sinus  is  increased  to  the  same  extent.  This  space,  how- 
ever, is  at  once  occupied  by  the  blood  from  the  branchiae, 
and  perhaps  by  some  blood  which  has  not  passed  through 
the  branchiae,  though  this  is  doubtful.  When  the  systole 
is  over,  the  diastole  follows ;  that  it  to  say,  the  elasticity 
of  the  walls  of  the  heart  and  that  of  the  various  parts 
which  connect  it  with  the  walls  of  the  pericardium,  bring 
it  back  to  its  former  size,  and  the  blood  in  the  pericardial 
sinus  flows  into  its  cavity  by  the  six  apertures.  With  a 
new  systole  the  same  process  is  repeated,  and  thus  the 
blood  is  driven  in  a  circular  course  through  all  parts  of 
the  body. 

It  will  be  observed  that  the  branchiae  are  placed  in  the 
course  of  the  current  of  blood  which  is  returning  to  the 
heart;  which  is  the  exact  contrary  of  what  happens  in 
fishes,  in  which  the  blood  is  sent  from  the  heart  to  the 
branchiae,  on  its  way  to  the  body.  It  follows,  from  this 
arrangement,  that  the  blood  which  goes  to  the  branchiae 
is  blood  in  which  the  quantity  of  oxygen  has  undergone 
a  diminution,  and  that  of  carbonic  acid  an  increase,  as 
compared  with  the  blood  in  the  heart  itself.  For  the 


THE   ORGANS    OF    RESPIRATION.  75 

activity  of  all  the  organs,  and  especially  of  the  muscles, 
is  inseparably  connected  with  the  absorption  of  oxygen 
and  the  evolution  of  carbonic  acid ;  and  the  only  source 
from  which  the  one  can  be  derived,  and  the  only  recep- 
tacle into  which  the  other  can  be  poured,  is  the  blood 
which  bathes  and  permeates  the  whole  fabric  to  which  it 
is  distributed  by  the  arteries. 

The  blood,  therefore,  which  reaches  the  branchiae  has 
lost  oxygen  and  gained  carbonic  acid ;  and  these  organs 
constitute  the  apparatus  for  the  elimination  of  the  inju- 
rious gas  from  the  economy  on  the  one  hand,  and,  on  the 
other,  for  the  taking  in  of  a  new  supply  of  the  needful 
"  vital  air,"  as  the  old  chemists  called  it.  It  is  thus  that 
the  branchiae  subserve  the  respiratory  function. 

The  crayfish  has  eighteen  perfect  and  two  rudimentary 
branchiae  in  each  branchial  chamber,  the  boundaries  of 
which  have  been  already  described. 

Of  the  eighteen  perfect  branchiae,  six  (podobranchice)  are 
attached  to  the  basal  joints  of  the  thoracic  limbs,  from  the 
last  but  one  to  the  second  (second  maxillipede)  inclusively 
(fig.  4.  p.  26,  pdb,  and  fig.  17,  A,  B) ;  and  eleven  (arthro- 
branchiai)  are  fixed  to  the  flexible  interarticular  mem- 
branes, which  connect  these  basal  joints  with  the  parts 
of  the  thorax  to  which  they  are  articulated  (fig.  4,  arb,  arb', 
fig.  17,  C).  Of  these  eleven  branchiae,  two  are  attached 
to  the  interarticular  membranes  of  all  the  ambulatory 
legs  but  the  last,  (=6)  and  to  those  of  the  pincers  and  of 
the  external  maxillipedes,  (=4)  and  one  to  that  of  the 


cxp-- 


PlG.  Yl.—Astacns  flnviatilis.—K,  one  of  the  podobranchiae  from  the 
outer  side  ;  B,  the  same  from  the  inner  side  ;  C,  one  of  the  arthro- 
branchise  ;  D,  a  part  of  one  of  the  coxopoditic  setae  ;  E,  extremity  of 
the  same  seta  ;  F,  extremity  of  a  seta  from  the  base  of  the  podo- 
branchia  ;  G,  hooked  seta  of  the  lamina ;  (A — C,  x  3  ;  D — G,  highly 
magnified).  Z>,  base  of  podobranchia ;  cs,  coxopoditic  seise ;  cxp, 
coxopodite  ;  Z,  lamina,  pi.  plume,  and  st,  stem  of  podobranchia  « 
t,  tubercle  on  the  coxopodite,  to  which  the  setse  are  attached. 


ARTHROBRANCHI^E  AND  PODOBRANCHLE.      77 

second  maxillipede.  The  first  maxillipede  and  the  last 
ambulatory  limb  have  none.  Moreover,  where  there  are 
two  arthrobranchiae,  one  is  more  or  less  in  front  of  and 
external  to  the  other. 

These  eleven  arthrobranchiae  are  all  very  similar  in 
structure  (fig.  17,  C).  Each  consists  of  a  stem  which  con- 
tains two  canals,  one  external  and  one  internal,  separated 
by  a  longitudinal  partition.  The  stem  is  beset  with  a  great 
number  of  delicate  branchial  filaments,  so  that  it  looks 
like  a  plume  tapering  from  its  base  to  its  summit.  Each 
filament  is  traversed  by  large  vascular  channels,  which 
break  up  into  a  net-work  immediately  beneath  the  surface. 
The  blood  driven  into  the  external  canals  of  the  stem  (fig. 
15,  av)  is  eventually  poured  into  the  inner  canal  (ev),  which 
again  communicates  with  the  channels  (bcv)  which  lead  to 
the  pericardial  sinus  (p).  In  its  course,  the  blood  traverses 
the  branchial  filaments,  the  outer  investment  of  each  of 
which  is  an  excessive^  thin  chitinous  membrane,  so  that 
the  blood  contained  in  them  is  practically  separated  by  a 
mere  film  from  the  aerated  water  in  which  the  gills  float. 
Hence,  an  exchange  of  gaseous  constituents  readily  takes 
place,  and  as  much  oxygen  is  taken  in  as  carbonic  acid  is 
given  out. 

The  six  podobranchise,  or  gills  which  are  attached  to 
the  basal  joints  of  the  legs,  play  the  same  part,  but  diffei 
a  good  deal  in  the  details  of  their  structure  from  those 
which  are  fixed  to  the  interarticular  membranes.  Each  con- 
sists of  a  broad  base  (fig.  17,  A  and  B  ;  b)  beset  with  many 


78        THE  PHYSIOLOGY   OF  THE   COMMON   CRAYFISH. 

fine  straight  hairs,  or  setae  (F),  whence  a  narrow  stem  (st) 
proceeds.  At  its  upper  end  this  stem  divides  into  two 
parts,  that  in  front,  the  plume  (pi),  resembling  the  free 
end  of  one  of  the  gills  just  described,  while  that  behind, 
the  lamina  (Q,  is  a  broad  thin  plate,  bent  upon  itself  longi- 
tudinally in  such  a  manner  that  its  folded  edge  lies  for- 
wards, and  covered  with  minute  hooked  setae  (G).  The 
gill  which  follows  is  received  into  the  space  included 
between  the  two  lobes  or  halves  of  the  folded  lamina 
(fig.  4,  p.  26).  Each  lobe  is  longitudinally  plaited  into 
about  a  dozen  folds.  The  whole  front  and  outer  face  of 
the  stem  is  beset  writh  branchial  filaments  ;  hence,  we  may 
compare  one  of  these  branchiae  to  one  of  the  preceding 
kind,  in  which  the  stem  has  become  modified  and  has 
given  off  a  large  folded  lamina  from  its  inner  and 
posterior  face. 

The  branchiaB  now  described  are  arranged  in  sets  of 
three  for  each  of  the  thoracic  limbs,  from  the  third 
rnaxillipede  to  the  last  but  one  ambulatory  limb,  and 
two  for  the  second  maxillipede,  thus  making  seventeen 
in  all  (3  X  5  +  2  =  17) ;  and,  between  every  two  there  is 
found  a  bundle  of  long  twisted  hairs  (fig.  17,  A,  cx.s;  D  and 
E),  which  are  attached  to  a  small  elevation  (t)  on  the  basal 
joint  of  each  limb.  These  coxopoditic  seta,  no  doubt,  serve 
to  prevent  the  intrusion  of  parasites  and  other  foreign 
matters  into  the  branchial  chamber.  From  the  mode 
of  attachment  of  the  six  branchiae  it  is  obvious  that  they 
must  share  in  the  movements  of  the  basal  joints  of  the 


PLEUROBRANCHLE,  COMPLETE  AND  RUDIMENTARY.  79 

legs ;  and  that,  when  the  crayfish  walks,  they  must  be 
more  or  less  agitated  in  the  branchial  chamber. 

The  eighteenth  branchia  resembles  one  of  the  eleven 
arthrobranchise  in  structure  ;  but  it  is  larger,  and  it  is 
attached  neither  to  the  basal  joint  of  the  hindermost  ambu- 
Utory  limb,  nor  to  its  interarticular  membrane,  but  to  the 
sides  of  the  thorax,  above  the  joint.  From  this  mode  of 
attachment  it  is  distinguished  from  the  others  as  apleuro- 
branchia  (fig.  4,  plb.  14). 

Finally,  in  front  of  this,  fixed  also  to  the  walls  of  the 
thorax,  above  each  of  the  two  preceding  pairs  of  ambulatory 
limbs,  there  is  a  delicate  filament,  about  a  sixteenth  of 
an  inch  long,  which  has  the  structure  of  a  branchial 
filament,  and  is,  in  fact,  a  rudimentary  pleurobranchia 
(fig.  4,  plb.  12,  plb.  13). 

The  quantity  of  water  which  occupies  the  space  left  in 
the  branchial  chamber  by  the  gills  is  but  small,  and  as 
the  respiratory  surface  offered  by  the  gills  is  relatively 
very  large,  the  air  contained  in  this  water  must  be 
rapidly  exhausted,  even  when  the  crayfish  is  quiescent ; 
while,  when  any  muscular  exertion  takes  place,  the  quan- 
tity of  carbonic  acid  formed,  and  the  demand  for  fresh 
oxygen,  is  at  once  greatly  increased.  For  the  efficient 
performance  of  the  function  of  respiration,  therefore,  the 
water  in  the  branchial  chamber  must  be  rapidly  renewed, 
and  there  must  be  some  arrangement  by  which  the 
supply  of  fresh  water  may  be  proportioned  to  the 
demand.  In  many  animals,  the  respiratory  surface  is 


60        THE  PHYSIOLOGY   OF  THE   COMMON   CRAYFISH. 

covered  with  rapidly  vibrating  filaments,  or  cilia,  by 
means  of  which  a  current  of  water  is  kept  con- 
tinually flowing  over  the  gills,  but  there  are  none  of  these 
in  the  crayfish.  The  same  object  is  attained,  however,  in 
another  way.  The  anterior  boundary  of  the  branchial 
chamber  corresponds  with  the  cervical  groove,  which,  as 
has  been  seen,  curves  downwards  and  then  forwards, 
until  it  terminates  at  the  sides  of  the  space  occupied  by 
the  jaws.  If  the  branchiostegite  is  cut  away  along  the 
groove,  it  will  be  found  that  it  is  attached  to  the  sides  of 
the  head,  which  project  a  little  beyond  the  anterior  part 
of  the  thorax,  so  that  there  is  a  depression  behind  the 
sides  of  the  head — just  as  there  is  a  depression,  behind  a 
man's  jaw,  at  the  sides  of  the  neck.  Between  this 
depression  in  front,  the  walls  of  the  thorax  internally, 
the  branchiostegite  externally,  and  the  bases  of  the  for- 
ceps and  external  foot-jaws  below,  a  curved  canal  is  in- 
cluded, by  which  the  branchial  cavity  opens  forwards  as  by 
a  funnel.  Attached  to  the  base  of  the  second  maxilla 
there  is  a  wide  curved  plate  (fig,  4,  6)  which  fits 
against  the  projection  of  the  head,  as  a  shirt  collar  might 
do,  to  carry  out  our  previous  comparison ;  and  this  scoop- 
shaped  plate  (termed  the  scaphognathite),  which  is  con- 
cave forwards  and  convex  backwards,  can  be  readily 
moved  backwards  and  forwards. 

If  a  living  crayfish  is  taken  out  of  the  water,  it  will 
be  found  that,  as  the  water  drains  away  from  the  branchial 
cavity,  bubbles  of  air  are  forced  out  of  its  anterior  opening. 


THE  RESPIRATORY   CURRENT.  81 

Again,  if,  when  a  crayfish  is  resting  quietly  in  the  water, 
a  little  coloured  fluid  is  allowed  to  run  down  towards 
the  posterior  opening  of  the  branchial  chamber,  it  will 
very  soon  be  driven  out  from  the  anterior  aperture, 
with  considerable  force,  in  a  long  stream.  In  fact,  as 
the  scaphognathite  vibrates  not  less  than  three  or  four 
times  in  a  second,  the  water  in  the  funnel-shaped  front 
passage  of  the  branchial  cavity  is  incessantly  baled  out ; 
and,  as  fresh  water  flows  in  from  behind  to  make  up  the 
loss,  a  current  is  kept  constantly  flowing  over  the  gills. 
The  rapidity  of  this  current  naturally  depends  on  the 
more  or  less  quick  repetition  of  the  strokes  of  the 
scaphognathite;  and  hence,  the  activity  of  the  respira- 
tory function  can  be  accurately  adjusted  to  the  wants  of 
the  economy.  Slow  working  of  the  scaphognathite 
answers  to  ordinary  breathing  in  ourselves,  quick  working 
to  panting. 

A  farther  self- adjustment  of  the  respiratory  apparatus 
is  gained  by  the  attachment  of  the  six  gills  to  the  basal 
joints  of  the  legs.  For,  when  the  animal  exerts  its 
muscles  in  walking,  these  gills  are  agitated,  and  thus  not 
only  bring  their  own  surfaces  more  largely  in  contact  with 
the  water,  but  produce  the  same  effect  upon  the  other 
gills. 

The  constant  oxidation  which  goes  on  in  all  parts  of 
the  body  not  only  gives  rise  to  carbonic  acid ;  but,  so  far 

as  it  affects  the  proteinaceous  constituents,  it  produces 

7 


82        THE   PHYSIOLOGY   OF   THE   COMMON   CRAYFISH. 

compounds  which  contain  nitrogen,  and  these,  like  other 
waste  products,  must  be  eliminated.  In  the  higher 
animals,  such  waste  products  take  the  form  of  urea,  uric 
acid,  hippuric  acid,  and  the  like;  and  are  got  rid  of  by 
the  kidneys.  We  may,  therefore,  expect  to  find  some 
organ  which  plays  the  part  of  a  kidney  in  the  crayfish  ; 
but  the  position  of  the  structure,  which  there  is  much 
reason  for  regarding  as  the  representative  of  the  kidney, 
is  so  singular  that  very  different  interpretations  have 
been  put  upon  it. 

On  the  basal  joint  of  each  antenna  it  is  easy  to  see  a 
small  conical  eminence  with  an  opening  on  the  inner  side 
of  its  summit  (fig.  18).  The  aperture  (x)  leads  by  a 
short  canal  into  a  spacious  sac,  with  extremely  delicate 
walls  (s),  which  is  lodged  in  the  front  part  of  the  head,  in 
front  of  and  below  the  cardiac  division  of  the  stomach  (cs). 
Beneath  this,  in  a  sort  of  recess,  which  corresponds  with 
the  epistoma,  and  with  the  base  of  the  antenna,  there  is  a 
discoidal  body  of  a  dull  green  colour,  in  shape  somewhat 
like  one  of  the  fruits  of  the  mallow,  which  is  known  as 
the  green  gland  (gg).  The  sac  narrows  below  like  a  wide 
funnel,  and  the  edges  of  its  small  end  are  continuous  with 
the  walls  of  the  green  gland ;  they  surround  an  aperture 
which  leads  into  the  interior  of  the  latter  structure,  and 
conveys  its  products  into  the  sac,  whence  they  are  excreted 
by  the  opening  in  the  antennary  papilla.  The  green  gland 
is  said  to  contain  a  substance  termed  guanin  (so  named 
because  it  is  found  in  the  guano  which  is  the  accumulated 


THE   RENAL  ORGAN. 


83 


excrement  of  birds),   a  nitrogenous  body  analogous  in 
some  respects  to  uric  acid,   but  less   highly  oxidated ; 


93 


FIG.  18.— Ast aei/s  fluriatilis.—k,  the  anterior  part  of  the  body,  with 
the  dorsal  portion  of  the  carapace  removed  to  show  the  position  of 
the  green  glands  ;  B,  the  same,  with  the  left  side  of  the  carapace 
removed  ;  C,  the  green  gland  removed  from  the  body  (all  x  2). 
(if/,  left  anterior  gastric  muscle  ;  c,  circumo3sophageal  commis- 
sures ;  cs,  cardiac  portion  of  stomach  ;  gg.  green  gland,  exposed 
in  A  on  the  left  side  by  the  removal  of  its  sac  ;  ima,  inter- 
maxillary or  cephalic  apodeme  ;  ces,  oesophagus  seen  in  transverse 
section  in  A,  the  stomach  being  removed  ;  s,  sac  of  green  gland  ; 
x,  bristle  passed  from  the  aperture  in  the  basal  joint  of  the  antenna 
into  the  sac. 

and  if  this  be  the  case,  there  can  be  little  doubt  that 
the  green  gland  represents  the  kidney,  and  its  secretion 


84         THE   PHYSIOLOGY   OF   THE   COMMON  CRAYFISH. 

the   urinary  fluid,   while   the    sac   is   a   sort   of  urinary 
bladder. 

Restricting  our  attention  to  the  phenomena  which  have 
now  heen  described,  and  to  a  short  period  in  the  life  of 
the  crayfish,  the  body  of  the  animal  may  be  regarded 
as  a  factory,  provided  with  various  pieces  of  machinery, 
by  means  of  which  certain  nitrogenous  and  other  matters 
are  extracted  from  the  animal  and  vegetable  substances 
which  serve  for  food,  are  oxidated,  and  are  then  delivered 
out  of  the  factory  in  the  shape  of  carbonic  acid  gas, 
guanin,  and  probably  some  other  products,  with  which 
we  are  at  present  unacquainted.  And  there  is  no  doubt, 
that  if  the  total  amount  of  products  given  out  could  be 
accurately  weighed  against  the  total  amount  of  materials 
taken  in,  the  weight  of  the  two  would  be  found  to  be 
identical.  To  put  the  matter  in  its  most  general  shape, 
the  body  of  the  crayfish  is  a  sort  of  focus  to  which  certain 
material  particles  converge,  in  which  they  move  for  a 
time,  and  from  which  they  are  afterwards  expelled  in  new 
combinations.  The  parallel  between  a  whirlpool  in  a 
stream  and  a  living  being,  which  has  often  been  drawn,  is 
as  just  as  it  is  striking.  The  whirlpool  is  permanent, 
but  the  particles  of  water  which  constitute  it  are  in- 
cessantly changing.  Those  which  enter  it,  on  the  one 
side,  are  whirled  around  and  temporarily  constitute  a  part 
of  its  individuality;  and  as  they  leave  it  on  the  other 
side,  their  places  are  made  good  by  new  comers. 


THE  WHIRLPOOL   OF  LIFE.  85 

Those  who  have  seen  the  wonderful  whirlpool,  three 
miles  below  the  Falls  of  Niagara,  will  not  have  forgotten 
the  heapod-up  wave  which  tumbles  and  tosses,  a  very 
embodiment  of  restless  energy,  where  the  swift  stream 
hurrying  from  the  Falls  is  compelled  to  make  a  sudden 
turn  towards  Lake  Ontario.  However  changeful  in  the 
contour  of  its  crest,  this  wave  has  been  visible,  approxi- 
mately in  the  same  place,  and  with  the  same  general 
form,  for  centuries  past.  Seen  from  a  mile  off,  it  would 
appear  to  be  a  stationary  hillock  of  water.  Viewed  closely, 
it  is  a  typical  expression  of  the  conflicting  impulses 
generated  by  a  swift  rush  of  material  particles. 

Now,  with  all  our  appliances,  we  cannot  get  within 
a  good  many  miles,  so  to  speak,  of  the  crayfish.  If  we 
could,  we  should  see  that  it  was  nothing  but  the  constant 
form  of  a  similar  turmoil  of  material  molecules  which 
are  constantly  flowing  into  the  animal  on  the  one  side, 
and  streaming  out  on  the  other. 

The  chemical  changes  which  take  place  in  the  body  of 
the  crayfish,  are  doubtless,  like  other  chemical  changes, 
accompanied  by  the  evolution  of  heat.  But  the  amount 
of  heat  thus  generated  is  so  small  and,  in  consequence 
of  the  conditions  under  which  the  crayfish  lives,  it  is  so 
easily  carried  away,  that  it  is  practically  insensible.  The 
crayfish  has  approximately  the  temperature  of  the  sur- 
rounding medium,  and  it  is,  therefore,  reckoned  among 
the  cold-blooded  animals. 

If  our  investigation  of  the  results  of  the  process  of 


86         THE  PHYSIOLOGY   OF  THE   COMMON  CRAYFISH. 

alimentation  in  a  well-fed  Crayfish  were  extended  over  a 
longer  time,  say  a  year  or  two,  we  should  find  that  the 
products  given  out  were  no  longer  equal  to  the  materials 
taken  in,  and  the  balance  would  be  found  in  the  increase 
of  the  animal's  weight.  If  we  inquired  how  the  balance 
was  distributed,  we  should  find  it  partly  in  store,  chiefly 
in  the  shape  of  fat ;  while,  in  part,  it  had  been  spent  in 
increasing  the  plant  and  in  enlarging  the  factory.  That 
is  to  say,  it  would  have  supplied  the  material  for  the 
animal's  growth.  And  this  is  one  of  the  most  remark- 
able respects  in  which  the  living  factory  differs  from 
those  which  we  construct.  It  not  only  enlarges  itself, 
but,  as  we  have  seen,  it  is  capable  of  executing  its  own 
repairs  to  a  very  considerable  extent. 


CHAPTER 

THE  PHYSIOLOGY  OF  THE  CRAYFISH — THE  MECHANISM  Bl 
WHICH  THE  LIVING  ORGANISM  ADJUSTS  ITSELF  TO 
SURROUNDING  CONDITIONS  AND  REPRODUCES  ITSELF. 

IF  the  hand  is  brought  near  a  vigorous  crayfish,  free 
to  move  in  a  large  vessel  of  water,  it  will  generally  give 
a  vigorous  flap  with  its  tail,  and  dart  backwards  out  of 
reach  ;  but  if  a  piece  of  meat  is  gently  lowered  into 
the  vessel,  the  crayfish  will  sooner  or  later  approach  and 
devour  it. 

If  we  ask  why  the  crayfish  behaves  in  this  fashion, 
every  one  has  an  answer  ready.  In  the  first  case,  it  is 
said  that  the  animal  is  aware  of  danger,  and  therefore 
hastens  away ;  in  the  second,  that  it  knows  that  meat  is 
good  to  eat,  and  therefore  walks  towards  it  and  makes  a 
meal.  And  nothing  can  seem  to  be  simpler  or  more 
satisfactory  than  these  replies,  until  we  attempt  to  con- 
ceive clearly  what  they  mean  ;  and,  then,  the  explanation, 
however  simple  it  may  be  admitted  to  be,  hardly  retains 
its  satisfactory  character. 

For  example,  when  we  say  that  the  crayfish  is,  "  aware 
of  danger,"  or  "  knows  that  meat  is  good  to  eat,"  what 


88        THE  PHYSIOLOGY  OF  THE  COMMON    CRAYFISH. 

do  we  mean  by  "being  aware"  and  "knowing"? 
Certainly  it  cannot  be  meant  that  the  crayfish  says  to 
himself,  as  we  do,  "This  is  dangerous,"  "That  is  nice  ;  " 
for  the  crayfish,  being  devoid  of  language,  has  nothing  to 
sa}'  either  to  himself  or  any  one  else.  And  if  the  cray- 
fish has  not  language  enough  to  construct  a  proposition, 
it  is  obviously  out  of  the  question  that  his  actions  should 
be  guided  by  a  logical  reasoning  process,  such  as  that 
by  which  a  man  would  justify  similar  actions.  The 
crayfish  assuredly  does  not  first  frame  the  syllogism, 
"  Dangerous  things  are  to  be  avoided ;  that  hand  is 
dangerous  ;  therefore  it  is  to  be  avoided ;  "  and  then  act 
upon  the  conclusion  thus  logically  drawn. 

But  it  may  be  said  that  children,  before  they  acquire 
the  use  of  language,  and  we  ourselves,  long  after  we  are 
familiar  with  conscious  reasoning,  perform  a  great  variety 
of  perfectly  rational  acts  unconsciously.  A  child  grasps 
at  a  sweetmeat,  or  cowers  before  a  threatening  gesture, 
before  it  can  speak  ;  and  any  one  of  us  would  start  back 
from  a  chasm  opening  at  our  feet,  or  stoop  to  pick  up  a 
jewel  from  the  ground,  "without  thinking  about  it." 
And,  no  doubt,  if  the  crayfish  has  any  mind  at  all,  his 
mental  operations  must  more  or  less  resemble  those  which 
the  human  mind  performs  without  giving  them  a  spoken 
or  unspoken  verbal  embodiment. 

If  we  analyse  these,  we  shall  find  that,  in  many  cases, 
distinctly  felt  sensations  are  followed  by  a  distinct  desire 
to  perform  some  act,  which  act  is  accordingly  performed  ; 


THE   CRAYFISH   MIXD.  89 

while,  in  other  cases,  the  act  follows  the  sensation  with- 
out one  being  aware  of  any  other  mental  process  ;  and, 
in  yet  others,  there  is  no  consciousness  even  of  the  sensa- 
tion. As  I  wrote  these  last  words,  for  example,  I  had 
not  the  slightest  consciousness  of  any  sensation  of  hold- 
ing or  guiding  the  pen,  although  my  fingers  were  caus- 
ing that  instrument  to  perform  exceedingly  complicated 
movements.  Moreover,  experiments  upon  animals  have 
proved  that  consciousness  is  wholly  unnecessary  to  the 
carrying  out  of  many  of  those  combined  movements  by 
which  the  body  is  adjusted  to  varying  external  conditions. 

Under  these  circumstances,  it  is  really  quite  an  open 
question  whether  a  crayfish  has  a  mind  or  not ;  more- 
over, the  problem  is  an  absolutely  insoluble  one,  inas- 
much as  nothing  short  of  being  a  crayfish  would  give  us 
positive  assurance  that  such  an  animal  possesses  con- 
sciousness ;  and,  finally,  supposing  the  crayfish  has  a 
mind,  that  fact  does  not  explain  its  acts,  but  only  shows 
that,  in  the  course  of  their  accomplishment,  they  are 
accompanied  by  phenomena  similar  to  those  of  which 
we  are  aware  in  ourselves,  under  like  circumstances. 

So  we  may  as  well  leave  this  question  of  the  crayfish's 
mind  on  one  side  for  the  present,  and  turn  to  a  more 
profitable  investigation,  namely,  that  of  the  order  and 
connexion  of  the  physical  phenomena  which  intervene 
between  something  which  happens  in  the  neighbourhood 
of  the  animal  and  that  other  something  which  responds 
to  it,  as  an  act  of  the  cra3Tfish. 


90         THE   PHYSIOLOGY    OF   THE   COMMON    CRAYFISH 

Whatever  else  it  may  be,  this  animal,  so  far  as  it  is 
acted  upon  by  bodies  around  it  and  reacts  on  them,  is  a 
piece  of  mechanism,  the  internal  works  of  which  give  rise 
to  certain  movements  when  it  is  affected  by  particular 
external  conditions  ;  and  they  do  this  in  virtue  of  their 
physical  properties  and  connexions. 

Every  movement  of  the  body,  or  of  any  organ  of  the 
body,  is  an  effect  of  one  and  the  same  cause,  namely, 
muscular  contraction.  Whether  the  crayfish  swims  or 
walks,  or  moves  its  antennae,  or  seizes  its  prey,  the  imme- 
diate cause  of  the  movements  of  the  parts  which  bring 
about,  or  constitute,  these  bodily  motions  is  to  be  sought 
in  a  change  which  takes  place  in  the  flesh,  or  muscle, 
which  is  attached  to  them.  The  change  of  place  which 
constitutes  any  movement  is  an  effect  of  a  previous 
change  in  the  disposition  of  the  molecules  of  one 
or  more  muscles ;  while  the  direction  of  that  move- 
ment depends  on  the  connexions  of  the  parts  of  the 
skeleton  with  one  another,  and  of  the  muscles  with 
them. 

The  muscle  of  the  crayfish  is  a  dense,  white  substance ; 
and  if  a  small  portion  of  it  is  subjected  to  examination  it 
will  be  found  to  be  very  easily  broken  up  into  more 
or  less  parallel  bundles  of  fine  fibres.  Each  of  these 
fibres  is  generally  found  to  be  ensheathed  in  a  fine  trans- 
parent membrane,  which  is  called  the  sarcolemma,  within 
which  is  contained  the  proper  substance  of  the  muscle. 
When  quite  fresh  and  living,  this  substance  is  soft  and 


THE  STRUCTURE   OF  MUSCLE. 


91 


semi-fluid,  but  it  hardens  and  becomes  solid  immediately 
after  death. 

Examined,    with    high    magnifying   powers,    in    this 


FIG.  19. — Afttacus  fuviatilis.—  A.  a  single  muscular  fibre;  transverse 
diameter  -^th  of  an  inch;  B,  a  portion  of  the  same  more  highly 
magnified ;  C,  a  smaller  portion  still  more  highly  magnified  ; 
D  and  E,  the  splitting  up  of  a  part  of  fibre  into  fibrillse ;  F,  the 
connexion  of  a  nervous  with  a  muscular  fibre  which  has  been 
treated  with  acetic  acid,  a,  darker,  and  &,  clearer  portions  of  the 
fibrillae  ;  n,  nucleus  of  sarcolemma  ;  nv,  nerve  fibre;  s,  sarcolemma; 
t,  tendon;  1 — 5,  successive  dark  bands  answering  to  the  darker 
portions,  a,  of  each  fibril  la. 


92        THE   PHYSIOLOGY  OF  THE   COMMON   CRAYFISH. 

condition,  the  muscle-substance  appears  marked  by  very 
regular  transverse  bands,  which  are  alternately  opaque 
and  transparent ;  and  it  is  characteristic  of  the  group  of 
animals  to  which  the  crayfish  belongs  that  their  muscle- 
substance  has  this  striped  character  in  all  parts  of  the 
body. 

A  greater  or  less  number  of  these  fibres,  united  into  one 
or  more  bundles,  constitutes  a  muscle  ;  and,  except  when 
these  muscles  surround  a  cavity,  they  are  fixed  at  each 
end  to  the  hard  parts  of  the  skeleton.  The  attachment 
is  frequently  effected  by  the  intermediation  of  a  dense, 
fibrous,  often  chitinous,  substance,  which  constitutes  the 
tendon  (fig.  19,  A;  t)  of  the  muscle. 

The  property  of  the  living  muscle,  which  enables  it  to 
be  the  cause  of  motion,  is  this  :  Every  muscular  fibre  is 
capable  of  suddenly  changing  its  dimensions,  in  such  a 
manner  that  it  shortens  and  becomes  proportionately 
thicker.  Hence  the  absolute  bulk  of  the  fibre  remains 
practically  unchanged.  From  this  circumstance,  muscular 
contraction,  as  the  -change  of  form  of  a  muscle  is  called, 
is  radically  different  from  the  process  which  commonly 
goes  by  the  same  name  in  other  things,  and  which 
involves  a  diminution  of  bulk. 

The  contraction  of  muscle  takes  place  with  great  force, 
and,  of  course,  if  the  parts  to  which  its  ends  are  fixed 
are  both  free  to  move,  they  are  brought  nearer  at  the 
moment  of  contraction  :  if  one  only  is  free  to  move  that 
is  approximated  to  the  fixed  part;  and  if  the  muscular 


MUSCLE   AS   THE   SOURCE   OF  MOTION. 


93 


fibre  surrounds  a  cavity,  the  cavity  is  lessened  when  the 
muscle  contracts.  This  is  the  whole  source  of  motor 
power  in  the  crayfish  machine.  The  results  produced 
by  the  exertion  of  that  power  depend  upon  the  manner 


prp. 


FIG.  20. — Axfitrttx  ttiiriatirm.—The  chela  of  the  forceps,  with,  one  side 
cut  a\vay  to  sho\v,  in  A,  the  muscles,  in  B,  the  tendons  (  x  2). 
cp,  carpopodite  ;  prp,  propodite  ;  dp,  dactylopodite  ;  m,  adductor 
muscle ;  //t',  abductor  muscle  ;  t,  tendon  of  adductor  muscle ;  ?, 
tendon  of  abductor  muscle  ;  a?,  hinge. 

in  which   the  parts   to  which  the  muscles  are   attached 
are  connected  with  one  another. 

One  example  of  this  has  already  been  given  in  the 
curious  mechanism  of  the  gastric  mill.  Another  may  be 
found  in  the  chela  which  terminates  the  forceps.  If  the 


94         THE   PHYSIOLOGY  OF   THE   COMMON   CRAYFISH. 

articulation  of  the  last  joint  (fig.  20,  dp)  with  the  one  which 
precedes  it  (prp)  is  examined,  it  will  be  found  that  the 
base  of  the  terminal  segment  (dp)  turns  on  two  hinges  (x), 
formed  by  the  hard  exoskeleton  and  situated  at  opposite 
points  of  the  diameter  of  the  base,  on  the  penultimate 
segment ;  and  these  hinges  are  so  disposed  that  the 
last  joint  can  be  moved  only  in  one  plane,  to  or  from 
the  produced  angle  of  the  penultimate  segment  (prp), 
which  forms  the  fixed  claw  of  the  chela.  Between  the 
hinges,  on  both  the  inner  and  the  outer  sides  of  the 
articulation,  the  exoskeleton  is  soft  and  flexible,  and 
allows  the  terminal  segment  to  play  easily  through  a 
certain  arc.  It  is  by  this  arrangement  that  the  direction 
and  the  extent  of  the  motion  of  the  free  claw  of  the  chela 
are  determined.  The  source  of  the  motion  lies  in  the 
muscles  which  occupy  the  interior  of  the  enlarged  penul- 
timate segment  of  the  limb.  Two  muscles,  one  of  very 
great  size  (m),  the  other  smaller  (w'),  are  fastened  by 
one  end  to  the  exoskeleton  of  this  segment.  The  fibres  of 
the  larger  muscle  converge  to  be  fixed  into  the  two  sides 
of  a  long  flat  process  of  the  chitinous  cuticula,  on  the 
inner  side  of  the  base  of  the  terminal  segment,  which 
serves  as  a  tendon  (t) ;  while  those  of  the  smaller  muscle 
are  similarly  attached  to  a  like  process  which  proceeds 
from  the  outer  side  of  the  base  of  the  terminal  seg- 
ment (£').  It  is  obvious  that,  when  the  latter  muscle 
shortens.it  must  move  the  apex  of  the  terminal  seg- 
ment (dp)  away  from  the  end  of  the  fixed  claw ;  while, 


MOTION  DIRECTED   BY  JOINTS.  95 

when  the  former  contracts,  the  end  of  the  terminal 
segment  is  brought  towards  that  of  the  fixed  claw. 

A  living  crayfish  is  able  to  perform  very  varied  move- 
ments with  its  pincers.  When  it  swims  backwards,  these 
limbs  are  stretched  straight  out,  parallel  with  one  another, 
in  front  of  the  head;  when  it  walks,  they  are  usually 
carried  like  arms  bent  at  the  elbow,  the  "  forearm " 
partly  resting  on  the  ground ;  on  being  irritated,  the 
crayfish  sweeps  the  pincers  round  in  any  direction  to 
grasp  the  offending  body ;  when  prey  is  seized,  it  is  at 
once  conveyed,  with  a  circular  motion,  towards  the  region 
of  the  mouth.  Nevertheless,  these  very  varied  actions 
are  all  brought  about  by  a  combination  of  simple  flexions 
and  extensions,  each  of  which  is  effected  in  the  exact 
order,  and  to  the  exact  extent,  needful  to  bring  the  chela 
into  the  position  required. 

The  skeleton  of  the  stem  of  the  limb  which  bears  the 
chela  is,  in  fact,  divided  into  four  moveable  segments ; 
and  each  of  these  is  articulated  with  the  segments  on 
each  side  of  it  by  a  hinge  of  just  the  same  character  as 
that  which  connects  the  moveable  claw  of  the  chela  with 
the  penultimate  segment,  while  the  basal  segment  is 
similarly  articulated  with  the  thorax. 

If  the  axes  of  all  these  articulations  *  were  parallel,  it  is 
obvious  that,  though  the  limb  might  be  moved  as  a  whole 
through  a  considerable  arc,  and  might  be  bent  in  various 

*  By  axis  of  the  articulation  is  meant  a  line  drawn  through  the  pail 
of  hinges  which  constitute  it. 


96        THE  PHYSIOLOGY   OF  THE   COMMON  CRAYFISH. 

degrees,  yet  all  its  movements  would  be  limited  to  one 
plane.  But,  in  fact,  the  axes  of  the  successive  articula- 
tions are  nearly  at  right  angles  to  one  another ;  so  that, 
if  the  segments  are  successively  either  extended  or 
flexed,  the  chela  describes  a  very  complicated  curve ; 
and  by  varying  the  extent  of  flexion  or  extension  of 
each  segment,  this  curve  is  susceptible  of  endless  varia- 
tion. It  would  probably  puzzle  a  good  mathematician 
to  say  exactly  what  position  should  be  given  to  each 
segment,  in  order  to  bring  the  chela  from  any  given 
position  into  any  other;  but  if  a  lively  crayfish  is 
incautiously  seized,  the  experimenter  will  find,  to  his 
cost,  that  the  animal  solves  the  problem  both  rapidly 
and  accurately. 

The  mechanism  by  which  the  retrograde  swimming  of 
the  crayfish  is  effected,  is  no  less  easily  analysed.  The 
apparatus  of  motion  is,  as  we  have  seen,  the  abdomen, 
with  its  terminal  five-pointed  flapper.  The  rings  of  the 
abdomen  are  articulated  together  by  joints  (fig.  21,  x  ) 
situated  a  little  below  the  middle  of  the  height  of  the 
rings,  at  opposite  ends  of  transverse  lines,  at  right 
angles  to  the  long  axis  of  the  abdomen. 

Each  ring  consists  of  a  dorsal,  arched  portion,  called  the 
tergum  (fig.  21 ;  fig.  36,  p.  142,  t.  XIX),  and  a  nearly  flat 
ventral  portion,  which  is  the  sternum  (fig.  36,  st.  XIX). 
Where  these  two  join,  a  broad  plate  is  sent  down  on 
each  side,  which  overlaps  the  bases  of  the  abdominal 
appendages,  and  is  known  as  ihepleuron  (fig.  36,  pi.  XIX). 


THE  JOINTS   OF  THE  ABDOMEN.  97 

The   sterna   are    all   very   narrow,    and    are    connected 
together  by  wide  spaces  of  flexible  exoskeleton. 

When  the  abdomen  is  made  straight,  it  will  be  found 
that  these  inter  sternal  membranes  are  stretched  as  far 
as  they  will  yield.  On  the  other  hand,  when  the  abdomen 


FlG.  21. — A#tacu*fluriatUix.— Two  of  the  abdominal  somites,  in  vertical 
section,  seen  from  the  inner  side,  to  show  x ,  x ,  the  hinges  by 
which  they  are  articulated  with  one  another  (  x  3).  The  anterior 
of  the  two  somites  is  that  to  the  right  of  the  figure. 

is  bent  up  as  far  as  it   will  go,  the  sterna  come  close 
together,  and  the  intersternal  membranes  are  folded. 

The  terga  are  very  broad ;  so  broad,  in  fact,  that  each 
overlaps  its  successor,  when  the  abdomen  is  straightened 
or  extended,  for  nearly  half  its  length  in  the  middle 
line ;  and  the  overlapped  surface  is  smooth,  convex,  and 


98    THE  PHYSIOLOGY  OF  THE  COMMON  CRAYFISH. 

marked  off  by  a  transverse  groove  from  the  rest  of  the 
tergum  as  an  articular  facet.  The  front  edge  of  the 
articular  facet  is  continued  into  a  sheet  of  flexible  cuti- 
cula,  which  turns  back,  and  passes  as  a  loose  fold  to  the 
hinder  edge  of  the  overlapping  tergum  (fig.  21).  This 
tergal  interarticular  membrane  allows  the  terga  to  move  as 
far  as  they  can  go  in  flexion ;  whilst,  in  extreme  exten- 
sion, they  are  but  slightly  stretched.  But,  even  if  the  in- 
tersternal  membranes  presented  no  obstacle  to  excessive 
extension  of  the  abdomen,  the  posterior  free  edge  of  each 
tergum  fits  into  the  groove  behind  the  facet  in  the  next 
in  such  a  manner,  that  the  abdomen  cannot  be  made  more 
than  very  slightly  concave  upwards  without  breaking. 

Thus  the  limits  of  motion  of  the  abdomen,  in  the 
vertical  direction,  are  from  the  position  in  which  it  is 
straight,  or  has  even  a  very  slight  upward  concavit}^  to 
that  in  which  it  is  completely  bent  upon  itself,  the  telson 
being  brought  under  the  bases  of  the  hinder  thoracic 
limbs.  No  lateral  movement  between  the  somites  of  the 
abdomen  is  possible  in  any  of  its  positions.  For,  when 
it  is  straight,  lateral  movement  is  hindered  not  only  by 
the  extensive  overlapping  of  the  terga,  but  also  by  the 
manner  in  which  the  hinder  edges  of  the  pleura  of  each 
of  the  four  middle  somites  overlap  the  front  edges  of 
their  successors.  The  pleura  of  the  second  somite  are 
much  larger  than  any  of  the  others,  and  their  front  edges 
overlap  the  small  pleura  of  the  first  abdominal  somite ; 
and  when  the  abdomen  is  much  flexed,  these  pleura  even 


THE  EXTENSORS  AND  FLEXORS  OF  THE  ABDOMEN.  99 

ride  oeer  the  posterior  edges  of  the  branchiostegites.  In 
the  position  of  extension,  the  overlap  of  the  terga  is  great, 
while  that  of  the  pleura  of  the  middle  somites  is  small. 
As  the  abdomen  passes  from  extension  to  flexion,  the 
overlap  of  the  terga  of  course  diminishes ;  but  any  de- 
crease of  resistance  to  lateral  strains  which  may  thus 
arise,  is  compensated  by  the  increasing  overlap  of  the 
pleura,  which  reaches  its  maximum  when  the  abdomen 
is  completely  flexed. 

It  is  obvious  that  longitudinal  muscular  fibres  fixed 
into  the  exoskeleton,  above  the  axes  of  the  joints,  must 
bring  the  centres  of  the  terga  of  the  somites  closer 
together,  when  they  contract ;  while  muscular  fibres 
attached  below  the  axes  of  the  joints  must  approximate 
the  sterna.  Hence,  the  former  will  give  rise  to  extension, 
and  the  latter  to  flexion,  of  the  abdomen  as  a  whole. 

Now  there  are  two  pairs  of  very  considerable  muscles 
disposed  in  this  manner.  The  dorsal  pair,  or  the  exten- 
sors of  the  abdomen  (fig.  22,  e.ra),  are  attached  in  front 
to  the  side  walls  of  the  thorax,  thence  pass  backwards 
into  the  abdomen,  and  divide  into  bundles,  which  are 
fixed  to  the  inner  surfaces  of  the  terga  of  all  the  somites. 
The  other  pair,  or  the  flexors  of  the  abdomen  (f.m)  consti- 
tute a  very  much  larger  mass  of  muscle,  the  fibres  of 
which  are  curiously  twisted,  like  the  strands  of  a  rope. 
The  front  end  of  this  double  rope  is  fixed  to  a  series  of 
processes  of  the  exoskeleton  of  the  thorax,  called  apode- 
mata,  some  of  which  roof  over  the  sternal  blood-sinusea 


100      THE   PHYSIOLOGY    OF   THE   COMMON    CRAYFISH. 

and  the  thoracic  part  of  the  nervous  S}rstem  ;  while,  in  the 
abdomen,  its  strands  are  attached  to  the  sternal  exoske-    / 
leton  of  all  the  somites  and  extend,  on  each  side  of  the 
rectum,  to  the  telson. 

When  the  exoskeleton  is   cleaned  by  maceration,  the 


add.m 


XVI       XV 


pep. 


FlG.  22. — Astacua  fur  tat  His.— A  longitudinal  section  of  the  body  to 
show  the  principal  muscles  and  their  relations  to  the  exoskeleton 
(nat.  size),  a,  the  vent ;  add.m,  adductor  muscle  of  mandible  ; 
e.m,  extensor,  and/.w.  flexor  muscle  of  abdomen  ;  #>*,  oesophagus  ; 
jjf/i,  procephalic  process  ;  t.t',  the  two  segments  of  the  telson  ; 
xv— xx,  the  abdominal  somites  ;  1—20,  the  appendages  ;  x  ,  x  , 
hinges  between  the  successive  abdominal  somites. 

abdomen  has  a  slight  curve,  dependent  upon  the  form  and 
the  degree  of  elasticity  possessed  by  its  different  parts; 
and,  in  a  living  crayfish  at  rest,  it  will  be  observed  that 
the  curvature  of  the  abdomen  is  still  more  marked. 
Hence  it  is  ready  either  for  extension  or  for  flexion. 

A  sudden  contraction  of  the  flexor  muscles  instantly 
increases    the    ventral    curvature    of  the    abdomen,   and 


THE  INFLUENCE  OF  NEfcVE   ON  MUSCLE  101 


throws  the  tail  fin,  the  two  side  lobes  of  which  are 
spread  out,  forwards ;  while  the  body  is  propelled  hack- 
wards  by  the  reaction  of  the  water  against  the  stroke. 
Then  the  flexor  muscles  being  relaxed,  the  extensor 
muscles  come  into  play ;  the  abdomen  is  straightened,  but 
less  violently  and  with  a  far  weaker  stroke  on  the  water, 
in  consequence  of  the  less  strength  of  the  extensors  and  of 
the  folding  up  of  the  lateral  plates  of  the  fin,  until  it 
comes  into  the  position  requisite  to  give  full  force  to  a 
new  downward  and  forward  stroke.  The  tendency  of  the 
extension  of  the  abdomen  is  to  drive  the  body  forward ; 
but  from  the  comparative  weakness  and  the  obliquity  of 
its  stroke,  its  practical  effect  is  little  more  than  to  check 
the  backward  motion  conferred  upon  the  body  by  the 
flexion  of  the  abdomen. 

Thus,  every  action  of  the  crayfish,  which  involves 
motion,  means  the  contraction  of  one  or  more  muscles. 
But  what  sets  muscle  contracting  ?  A  muscle  freshly 
removed  from  the  body  may  be  made  to  contract  in 
various  wa}rs,  as  by  mechanical  or  chemical  irritation,  or 
by  an  electrical  shock ;  but,  under  natural  conditions, 
there  is  only  one  cause  of  muscular  contraction,  and  that 
is  the  activity  of  a  nerve.  Every  muscle  is  supplied  with 
one  or  more  nerves.  These  are  delicate  threads  which, 
on  microscopic  examination,  prove  to  be  bundles  of  fine 
tubular  filaments,  filled  with  an  apparently  structureless 
substance  of  gelatinous  consistency,  the  nerve  fibres 


102      THE   PHYSIOLOGY   GF   THE   COMMON    CRAYFISH. 

(fig.  23).  The  nerve  bundle  which  passes  to  a  muscle 
breaks  up  into  smaller  bundles  and,  finally,  into  separate 
fibres,  each  of  which  ultimately  terminates  by  becoming 
continuous  with  the  substance  of  a  muscular  fibre  fig.  19, 
F.)  Now  the  peculiarity  of  a  muscle  nerve,  or  motor 
nerve,  as  it  is  called,  is  that  irritation  of  the  nerve  fibre  at 
any  part  of  its  length,  however  distant  from  the  muscle, 


FlG.  23.—  AstaciiK  fluriatilis. — Three  nerve  fibres,  with  the  connective 
tissue  in  which  they  are  imbedded.  (Magnified  about  250  dia- 
meters.) n,  nuclei. 

brings  about  muscular  contraction,  just  as  if  the  muscle 
itself  were  irritated.  A  change  is  produced  in  the  mole- 
cular condition  of  the  nerve  at  the  point  of  irritation ; 
and  this  change  is  propagated  along  the  nerve,  until  it 
reaches  the  muscle,  in  which  it  gives  rise  to  that  change 
in  the  arrangement  of  its  molecules,  the  most  obvious 
effect  of  which  is  the  sudden  alteration  of  form  which  we 
call  muscular  contraction. 

If  we  follow  the   course    of  the    motor  nerves   in   a 


NERVE  FIBRES  AND  NERVE  CELLS. 


103 


direction  away  from  the  muscles  to  which  they  are  dis- 
tributed, they  will  be  found,  sooner  or  later,  to  terminate 
in  ganglia  (fig.  24  A.  gl.c ;  fig.  25,  gn.  1 — 13.)  A  gan- 
glion is  a  body  which  is  in  great  measure  composed  of 


B 


FIG.  24. —  AstacugflHi'latilis.—b.,  one  of  the  (double)  abdominal  gan- 
glia, with  the  nerves  connected  with  it  (  x  25)  ;  B,  a  nerve  cell  or 
ganglionic  corpuscle  (  x  250).  a,  sheath  of  the  nerves  ;  c,  sheath 
of  the  ganglion  ;  co,  co',  commissural  cords  connecting  the  ganglia 
with  those  in  front,  and  those  behind  them.  gl.c.  points  to  the 
ganglionic  corpuscles  of  the  ganglia  ;  »,  nerve  fibres. 

nerve  fibres  ;  but,  interspersed  among  these,  or  disposed 
around  them,  there  are  peculiar  structures,  which  are 
termed  ganglionic  corpuscles,  or  nerve  cells  (fig.  24,  B.) 
These  are  nucleated  cells,  not  unlike  the  epithelial  cells 
which  have  been  already  mentioned,  but  which  are  larger 


FlG.  25.—Astacusjlnviatilis.—The  central  nervous  system  seen  from 
above  (nat.  size),  a,  vent ;  an,  antennary  nerve  ;  a'n,  antennulary 
nerve;  c,  circumcesophageal  commissures  ;  gn.  1,  supraoesophageal 
ganglion ;  gn.  2,  infraoesophageal  ganglion  ;  gn.  (!,  fifth  thoracic 
ganglion  ;  gn.  7,  last  thoracic  ganglion  ;  gn.  13,  last  abdominal  gang- 
lion  ;  oes,  oesophagus  in  cross  section  ;  on,  optic  nerve;  sa,  sternal 
artery  in  cross  section  ;  sgn,  stomatogastric  nerve. 


THE   CHAIN   OF  GANGLIA.  105 

and  often  give  off  one  or  more  processes.  These  pro- 
cesses, under  favourable  circumstances,  can  be  traced 
into  continuity  with  nerve  fibres. 

The  chief  ganglia  of  the  crayfish  are  disposed  in  a 
longitudinal  series  in  the  middle  line  of  the  ventral 
aspect  of  the  body  close  to  the  integument  (fig.  25). 
In  the  abdomen,  for  example,  six  ganglionic  masses  are 
readily  observed,  one  lying  over  the  sternum  of  each 
somite,  connected  by  longitudinal  bands  of  nerve  fibres, 
and  giving  off  branches  to  the  muscles.  On  careful  ex- 
amination, the  longitudinal  connecting  bands,  or  com- 
missures (fig.  24,  co),  are  seen  to  be  double,  and  each 
mass  appears  slightly  bilobed.  In  the  thorax,  there  are 
six,  larger,  double  ganglionic  masses,  likewise  connected 
by  double  commissures  ;  and  the  most  anterior  of  these, 
which  is  the  largest  (fig.  25,  gn.  2),  is  marked  at  the 
sides  by  notches,  as  if  it  were  made  up  of  several  pairs 
of  ganglia,  run  together  into  one  continuous  whole. 
In  front  of  this,  two  commissures  (c)  pass  forwards, 
separating  widely,  to  give  room  for  the  gullet  (ces),  which 
passes  between  them ;  while  in  front  of  the  gullet,  just 
behind  the  eyes,  they  unite  with  a  transversely  elongated 
mass  of  ganglionic  substance  (gn.  1),  termed  the  brain,  or 
cerebral  ganglion. 

All  the  motor  nerves,  as  has  been  said,  are  traceable, 
directly  or  indirectly,  to  one  or  other  of  these  thirteen 
sets  of  ganglia ;  but  other  nerves  are  given  off  from  the 
ganglia,  which  cannot  be  followed  into  any  muscle.  In 


106      THE   PHYSIOLOGY   OF  THE  COMMON  CRAYFISH. 

fact,  these  nerves  go  either  to  the  integument  or  to  the 
organs  of  sense,  and  they  are  termed  sensory  nerves. 

When  a  muscle  is  connected  by  its  motor  nerve  with 
a  ganglion,  irritation  of  that  ganglion  will  bring  about 
the  contraction  of  the  muscle,  as  well  as  if  the  motor 
nerve  itself  were  irritated.  Not  only  so  ;  but  if  a  sensory 
nerve,  which  is  in  connexion  with  the  ganglion,  is  irritated, 
the  same  effect  is  produced  ;  moreover,  the  sensory  nerve 
itself  need  not  be  excited,  but  the  same  result  will 
take  place,  if  the  organ  to  which  it  is  distributed  is 
stimulated.  Thus  the  nervous  system  is  fundamentally 
an  apparatus  by  which  two  separate,  and  it  may  be  dis- 
tant, parts  of  the  body,  are  brought  into  relation  with 
one  another ;  and  this  relation  is  of  such  a  nature,  that 
a  change  of  state  arising  in  the  one  part  is  followed  by 
the  propagation  of  changes  along  the  sensory  nerve  to  the 
ganglion,  and  from  the  ganglion  to  the  other  part ;  where, 
if  that  part  happens  to  be  muscle,  it  produces  contraction. 
If  one  end  of  a  rod  of  wood,  twenty  feet  long,  is  applied 
to  a  sounding-board,  the  sound  of  a  tuning-fork  held 
against  the  opposite  extremity  will  be  very  plainly  heard. 
Nothing  can  be  seen  to  happen  in  the  wood,  and  yet 
its  molecules  are  certainly  set  vibrating,  at  the  same 
rate  as  the  tuning-fork  vibrates ;  and  when,  after 
travelling  rapidly  along  the  wood,  these  vibrations 
affect  the  sounding-board,  they  give  rise  to  vibrations 
of  the  molecules  of  the  air,  which  reaching  the  ear,  are 
converted  into  an  audible  note.  So  in  the  nerve  tract : 


THE  CO-ORDINATION   OF  MOVEMENTS.  107 

no  apparent  change  is  effected  in  it  by  the  irritation  at 
one  end ;  but  the  rate  at  which  the  molecular  change 
produced  travels  can  be  measured  ;  and,  when  it  reaches 
the  muscle,  its  effect  becomes  visible  in  the  change  of 
form  of  the  muscle.  The  molecular  change  would  take 
place  just  as  much  if  there  were  no  muscle  connected 
with  the  nerve,  but  it  would  be  no  more  apparent  to 
ordinary  observation  than  the  sound  of  the  tuning-fork 
is  audible  in  the  absence  of  the  sounding-board. 

If  the  nervous  system  were  a  mere  bundle  of  nerve 
fibres  extending  between  sensory  organs  and  muscles, 
every  muscular  contraction  would  require  the  stimulation 
of  that  special  point  of  the  surface  on  which  the  appro- 
priate sensory  nerve  ended.  The  contraction  of  several 
muscles  at  the  same  time,  that  is,  the  combination  of 
movements  towards  one  end,  would  be  possible  only  if  the 
appropriate  nerves  were  severally  stimulated  in  the  proper 
order,  and  every  movement  would  be  the  direct  result  of  ex- 
ternal changes.  The  organism  would  be  like  a  piano,  which 
may  be  made  to  give  out  the  most  complicated  harmonies, 
but  is  dependent  for  their  production  on  the  depression 
of  a  separate  key  for  every  note  that  is  sounded.  But  it 
is  obvious  that  the  crayfish  needs  no  such  separate 
impulses  for  the  performance  of  highly  complicated 
actions.  The  simple  impression  made  on  the  organs  of 
sensation  in  the  two  examples  with  which  we  started, 
gives  rise  to  a  train  of  complicated  and  accurately  co- 
ordinated muscular  contractions.  To  carry  the  analogy 


108      THE  PHYSIOLOGY   OF  THE   COMMON   CRAYFISH. 

of  the  musical  instrument  further,  striking  a  single  key 
gives  rise,  not  to  a  single  note,  but  to  a  more  or  less 
elaborate  tune ;  as  if  the  hammer  struck  not  a  single 
string,  but  pressed  down  the  stop  of  a  musical  box. 

It  is  in  the  ganglia  that  we  must  look  for  the  analogue 
of  the  musical  box.  A  single  impulse  conveyed  by  a 
sensory  nerve  to  a  ganglion,  may  give  rise  to  a  single 
muscular  contraction,  but  more  commonly  it  originates  a 
series  of  such,  combined  to  a  definite  end. 

The  effect  which  results  from  the  propagation  of  an 
impulse  along  a  nerve  fibre  to  a  ganglionic  centre,  whence 
it  is,  as  it  were,  reflected  along  another  nerve  fibre  to  a 
muscle,  is  what  is  termed  a  reflex  action.  As  it  is  by  no 
means  necessary  that  sensation  should  be  a  concomitant 
of  the  first  impulse,  it  is  better  to  term  the  nerve  fibre 
which  carries  it  afferent  rather  than  sensory  ;  and,  as 
other  phenomena  besides  those  of  molar  motion  may  be 
the  ultimate  result  of  the  reflex  action,  it  is  better  to 
term  the  nerve  fibre  which  transmits  the  reflected  im- 
pulse efferent  rather  than  motor. 

If  the  nervous  commissures  between  the  last  thoracic 
and  the  first  abdominal  ganglia  are  cut,  or  if  the  thoracic 
ganglia  are  destroj^ed,  the  crayfish  is  no  longer  able  to 
control  the  movements  of  the  abdomen.  If  the  forepart 
of  the  body  is  irritated,  for  example,  the  animal  makes 
nc  effort  to  escape  by  swimming  backwards.  Never- 
theless, the  abdomen  is  not  paralysed,  for,  if  it  be  irri- 
tated, it  will  flap  vigorously.  This  is  a  case  of  pure 


INVOLUNTARY   RHYTHMICAL  MOVEMENTS.  109 

reflex  action.  The  stimulus  is  conveyed  to  the  abdo- 
minal ganglia  through  afferent  nerves,  and  is  reflected 
from  them,  by  efferent  nerves,  to  the  abdominal  muscles. 

But  this  is  not  all.  Under  these  circumstances  it  will 
be  seen  that  the  abdominal  limbs  all  swing  backwards 
and  forwards,  simultaneously,  with  an  even  stroke ;  while 
the  vent  opens  and  shuts  with  a  regular  rhythm.  Of 
course,  these  movements  imply  correspondingly  regular 
alternate  contractions  and  relaxations  of  certain  sets  of 
muscles ;  and  these,  again,  imply  regularly  recurring 
efferent  impulses  from  the -abdominal  ganglia.  The  fact 
that  these  impulses  proceed  from  the  abdominal  ganglia, 
may  be  shown  in  two  ways :  first,  by  destroying  these 
ganglia  in  one  somite  after  another,  when  the  move- 
ments in  each  somite  at  once  permanently  cease ;  and, 
secondly,  by  irritating  the  surface  of  the  abdomen,  when 
the  movements  are  temporarily  inhibited  by  the  stimula- 
tion of  the  afferent  nerves.  Whether  these  movements  are 
properly  reflex,  that  is,  arise  from  incessant  new  afferent 
impulses  of  unknown  origin,  or  whether  they  depend  on  the 
periodical  accumulation  and  discharge  of  nervous  energy  in 
the  ganglia  themselves,  or  upon  periodical  exhaustion  and 
restoration  of  the  irritability  of  the  muscles,  is  unknown. 
It  is  sufficient  for  the  present  purpose  to  use  the  facts  as 
tvideiice  of  the  peculiar  co-ordinative  function  of  ganglia. 

The  crayfish,  as  we  have  seen,  avoids  light;  and  the 
slightest  touch  of  one  of  its  antennae  gives  rise  to  active 
j  of  the  whole  body.  In  fact,  the  animal's  posi- 


110      THE  PHYSIOLOGY   OF  THE  COMMON   CRAYFISH. 

tion  and  movements  are  largely  determined  by  the  in« 
fluences  received  through  the  feelers  and  the  eyes.  These 
receive  their  nerves  from  the  cerebral  ganglia ;  and,  as 
might  be  expected,  when  these  ganglia  are  extirpated, 
the  crayfish  exhibits  no  tendency  to  get  away  from  the 
light,  and  the  feelers  may  not  only  be  touched,  but 
sharply  pinched,  without  effect.  Clearly,  therefore,  the 
cerebral  ganglia  serve  as  a  ganglionic  centre,  by  which 
the  afferent  impulses  derived  from  the  ieelers  and  the 
eyes  are  transmuted  into  efferent  impulses.  Another 
very  curious  result  follows  upon  the  extirpation  of  the 
cerebral  ganglia.  If  an  uninjured  cra}Tfish  is  placed  upon 
its  back,  it  makes  unceasing  and  well-directed  efforts  to 
turn  over ;  and  if  everything  else  fails,  it  will  give  a 
powerful  flap  with  the  abdomen,  and  trust  to  the  chapter 
of  accidents  to  turn  over  as  it  darts  back.  But  the 
brainless  crayfish  behaves  in  a  very  different  way.  Its 
limbs  are  in  incessant  motion,  but  they  are  "  all  abroad ;  " 
and  if  it  turns  over  on  one  side,  it  does  not  seem  able 
to  steady  itself,  but  rolls  on  to  its  back  again. 

If  anything  is  put  between  the  chelae  of  an  uninjured 
crayfish,  while  on  its  back,  it  either  rejects  the  object  at 
once,  or  tries  to  make  use  of  it  for  leverage  to  turn  over. 
In  the  brainless  crayfish  a  similar  operation  gives  rise  to 
a  very  curious  spectacle.*  If  the  object,  whatever  it  be 

*  My  attention  was  first  drawn  to  these  phenomena  by  my  friend 
Dr.  M.  Fosler,  F.R.S.,  to  whom  I  had  suggested  the  desirableness  of 
an  experimental  study  of  the  nerve  physiology  of  the  crayfish. 


THE  ACTIONS  OF  BRAINLESS  CRAYFISHES.  Ill 

bit  of  metal,  or  wood,  or  paper,  or  one  of  the  ani- 
mal's own  antennae — is  placed  between  the  chelae  of  the 
forceps,  it  is  at  once  seized  by  them,  and  carried  back- 
wards ;  the  chelate  ambulatory  limbs  are  at  the  same 
tims  advanced,  the  object  seized  is  transferred  to  them, 
and  they  at  once  tuck  it  between  the  external  maxilli- 
pedes,  which,  with  the  other  jaws,  begin  vigorously  to 
masticate  it.  Sometimes  the  morsel  is  swallowed; 
sometimes  it  passes  out  between  the  anterior  jaws,  as  if 
deglutition  were  difficult.  It  is  very  singular  to  observe 
that,  if  the  morsel  which  is  being  conveyed  to  the  mouth 
by  one  of  the  forceps  is  pulled  back,  the  forceps  and  the 
chelate  ambulatory  limbs  of  the  other  side  are  at  once 
brought  forward  to  secure  it.  The  movements  of  the 
limbs  are,  in  short,  adjusted  to  meet  the  increased 
resistance. 

All  these  phenomena  cease  at  once,  if  the  thoracic 
ganglia  are  destroyed.  It  is  in  these,  therefore,  that  the 
simple  stimulus  set  up  by  the  contact  of  a  body  with,  for 
example,  one  of  the  forceps,  is  translated  into  all  the  sur- 
prisingly complex  and  accurately  co-ordinated  movements, 
which  have  been  described.  Thus  the  nervous  system 
of  the  crayfish  may  be  regarded  as  a  system  of  co-ordi- 
nating mechanisms,  each  of  which  produces  a  certain 
action,  or  set  of  actions,  on  the  receipt  of  an  appropriate 
stimulus. 

AVhen  the  crayfish  conies  into  the  world,  it  possesses 
in  its  neuro-muscular  apparatus  certain  innate  poten- 


112       THE  PHYSIOLOGY  OF  THE   COMMON   CRAYFISH. 

tialities  of  action,  and  will  exhibit  the  corresponding 
acts,  under  the  influence  of  the  appropriate  stimuli. 
A  large  proportion  of  these  stimuli  come  from  without 
tlnough  the  organs  of  the  senses.  The  greater  or  less 
readiness  of  each  sense  organ  to  receive  impulses,  of 
the  nerves  to  transmit  them,  and  of  the  ganglia  to 
give  rise  to  combined  impulses,  is  dependent  at  any 
moment  upon  the  physical  condition  of  these  parts ;  and 
this,  again,  is  largely  modified  by  the  amount  and  the 
condition  of  the  blood  supplied.  On  the  other  hand,  a 
certain  number  of  these  stimuli  are  doubtless  originated 
by  changes  within  the  various  organs  which  compose  the 
body,  including  the  nerve  centres  themselves. 

When  an  action  arises  from  conditions  developed  in 
the  interior  of  an  animal's  body,  inasmuch  as  we  cannot 
perceive  the  antecedent  phenomena,  we  call  such  an 
action  "spontaneous;"  or,  when  in  ourselves  we  are 
aware  that  it  is  accompanied  by  the  idea  of  the  action, 
and  the  desire  to  perform  it,  we  term  the  act  "  volun- 
tary." But,  by  the  use  of  this  language,  no  rational 
person  intends  to  express  the  belief  that  such  acts  are 
uncaused  or  cause  themselves.  "  Self-causation "  is  a 
contradiction  in  terms ;  and  the  notion  that  any  pheno- 
menon comes  into  existence  without  a  cause,  is  equivalent 
to  a  belief  in  chance,  which  one  may  hope  is,  by  this 
time,  finally  exploded. 

In  the  crayfish,  at  any  rate,  there  is  not  the  slightest 
reason  to  doubt  that  every  action  has  its  definite  physical 


SENSORY   ORGAXS.  113 

cause,  and  that  what  it  does  at  any  moment  would  be 
as  clearly  intelligible,  if  we  only  knew  all  the  internal 
and  external  conditions  of  the  case,  as  the  striking  of  a 
clock  is  to  any  one  who  understands  clockwork. 

The  adjustment  of  the  body  to  varying  external  con- 
ditions, which  is  one  of  the  chief  results  of  the  working 
of  the  nervous  mechanism,  would  be  far  less  important 
from  a  physiological  point  of  view  than  it  is,  if  only 
those  external  bodies  which  come  into  direct  contact 
with  the  organism  *  could  affect  it ;  though  very  delicate 
influences  of  this  kind  take  effect  on  the  nervous  apparatus 
through  the  integument. 

It  is  probable  that  the  seta,  or  hairs,  which  are  so 
generally  scattered  over  the  body  and  the  appendages, 
are  delicate  tactile  organs.  They  are  hollow  processes  of 
the  chitinous  cuticle,  and  their  cavities  are  continuous 
with  narrow  canals,  which  traverse  the  whole  thick- 
ness of  the  cuticle,  and  are  filled  by  a  prolongation  of 
the  subjacent  proper  integument.  As  this  is  supplied 
with  nerves,  it  is  likely  that  fine  nerve  fibres  reach  the 
bases  of  the  hairs,  and  are  affected  by  anything  which 
stirs  these  delicately  poised  levers. 

*  It  may  be  said  that,  strictly  speaking,  only  those  external  bodies 
\vhich  are  in  direct  contact  with  the  organism  do  affect  it — as  the 
vibrating  ether,  in  the  case  of  luminous  bodies  ;  the  vibrating  air  or 
water,  in  the  case  of  sonorous  bodies  ;  odorous  particles,  in  the  case  of 
odorous  bodies  :  but  I  have  preferred  the  ordinary  phraseology  to  a 
pedantically  accurate  periphrasis. 

9 


THE   PHYSIOLOGY   OF   THE   COMMON   CRAYFISH. 

There  is  much  reason  to  believe  that  odorous  bodies 
affect  crayfish  ;  but  it  is  very  difficult  to  obtain  experi- 


FlG.  26. — Astaciis  flwviatilis. — A,  the  right  antennule  seen  from  the 
inner  side  (  x  5)  ;  B,  a  portion  of  the  exopodite  enlarged  ;  C,  olfactory 
appendage  of  the  exopodite  ;  a,  front  view  ;  b,  side  view  (  x  300) ; 
a,  olfactory  appendages ;  au,  auditory  sac.  supposed  to  be  seen  through 
the  wall  of  the  basal  joint  of  the  antennule  ;  b,  setae  ;  en,  endopo- 
dite  ;  ex,  exopodite  ;  sp.  spine  of  the  basal  joint. 

mental  evidence  of  the  fact.  However,  there  is  a  good 
deal  of  analogical  ground  for  the  supposition  that  some 
peculiar  structures,  which  are  evidently  of  a  sensory 


THE  "OLFACTORY   ORGANS.  115 

nature,  developed  on  the  under  side  of  the  outer  branch 
of  the  antennule,  play  the  part  of  an  olfactory  apparatus. 

Both  the  outer  (fig.  26  A.  ex)  and  the  inner  (en) 
branches  of  the  antennule  are  made  up  of  a  number  of 
delicate  ring-like  segments,  which  bear  fine  setae  (b)  of 
the  ordinary  character. 

The  inner  branch,  which  is  the  shorter  of  the  two,  pos- 
sesses only  these  setae ;  but  the  under  surface  of  each  of 
the  joints  of  the  outer  branch,  from  about  the  seventh  or 
eighth  to  the  last  but  one,  is  provided  with  two  bundles 
of  very  curious  appendages  (fig.  27,  A,  B,  C,  a),  one  in 
front  and  one  behind.  These  are  rather  more  than 
1 -200th  of  an  inch  long,  very  delicate,  and  shaped  like  a 
spatula,  with  a  rounded  handle  and  a  flattened  somewhat 
curved  blade,  the  end  of  which  is  sometimes  truncated, 
sometimes  has  the  form  of  a  prominent  papilla.  There 
is  a  sort  of  joint  between  the  handle  and  the  blade,  such 
as  is  found  between  the  basal  and  the  terminal  parts  of 
the  ordinary  setae,  with  which,  in  fact,  these  processes 
entirely  correspond  in  their  essential  structure.  A  soft 
granular  tissue  fills  the  interior  of  each  of  these  pro- 
blematical structures,  to  which  Leydig,  their  discoverer, 
ascribes  an  olfactory  function. 

It  is  probable  that  the  crayfish  possesses  something 
analogous  to  taste,  and  a  very  likely  seat  for  the  organ 
of  this  function  is  in  the  upper  lip  and  the  metastoma; 
but  if  the  organ  exists  it  possesses  no  structural  pecu- 
liarities by  which  it  can  be  identified. 


116      THE  PHYSIOLOGY  OF  THE   COMMON   CRAYFISH. 

There  is  no  doubt,  however,  as  to  the  special  recipients 
of  sonorous  and  luminous  vibrations;  and  these  are  of 
particular  importance,  as  they  enable  the  nervous  ma- 
chinery to  be  affected  by  bodies  indefinitely  remote 
from  it,  and  to  change  the  place  of  the  organism  in 
relation  to  such  bodies. 

Sonorous  vibrations  are  enabled  to  act  as  the  stimulants 
of  a  special  nerve  (fig.  25,  an)  connected  with  the  brain, 
by  means  of  the  very  curious  auditory  sacs  (fig.  26,  A,  au) 
which  are  lodged  in  the  basal  joints  of  the  antennules. 

Each  of  these  joints  is  trihedral,  the  outer  face  being  con- 
vex ;  the  inner,  applied  to  its  fellow,  flat ;  and  the  upper, 
on  which  the  eyestalk  rests,  concave.  On  this  upper  face 
there  is  a  narrow  elongated  oval  aperture,  the  outer  lip  oi 
which  is  beset  with  a  flat  brush  of  long  close-set  setae, 
which  lie  horizontally  over  the  aperture,  and  effectually 
close  it.  The  aperture  leads  into  a  small  sac  (au)  with 
delicate  walls  formed  by  a  chitinous  continuation  of  the 
general  cuticula.  The  inferior  and  posterior  wall  of  the 
sac  is  raised  up  along  a  curved  line  into  a  ridge  which 
projects  into  its  interior  (fig.  27,  A,  r).  Each  side  of  this 
ridge  is  beset  with  a  series  of  delicate  setae  (as),  the 
longest  of  which  measures  about  -^th  of  an  inch ;  they 
thus  form  a  longitudinal  band  bent  upon  itself.  These 
auditory  setae  project  into  the  fluid  contents  of  the  sac, 
and  their  apices  are  for  the  most  part  imbedded  in  a 
gelatinous  mass,  which  contains  irregular  particles  of  sand 


THE   EAR  OF   THE   CRAYFISH. 


117 


and  sometimes  of  other  foreign  matter.  A  nerve  (n  n ,)  is 
distributed  to  the  sac,  and  its  fibres  enter  the  bases  of 
the  hairs,  and  may  be  traced  to  their  apices,  where  they 
end  in  peculiar  elongated  rod-like  bodies  (fig.  27,  C). 
Here  is  an  auditory  organ  of  the  simplest  description. 


FIG.  27. — Astacus  fliiviatilis.  A,  the  auditory  sac  detached  and  seen 
from  the  outside  (  x  15)  ;  B,  auditory  hair  (  x  100)  ;  C,  the  distal  ex- 
tremity of  the  same  more  highly  magnified,  a,  aperture  of  sac  ;  as, 
auditory  setae  ;  b,  its  inner  or  posterior  extremity  ;  n  n',  nerves  ; 
?•,  ridge. 

It  retains,  in  fact,  throughout  life,  the  condition  of  a 
simple  sac  or  involution  of  the  integument,  such  as  is 
that  of  the  vertebrate  ear  in  its  earliest  stage. 


118      THE   PHYSIOLOGY   OF  THE   COMMON   CRAYFISH. 

The  sonorous  vibrations  transmitted  through  the 
water  in  which  the  crayfish  lives  to  the  fluid  and  solid 
contents  of  the  auditory  sac  are  taken  up  by  the  delicate 
hairs  of  the  ridge,  and  give  rise  to  molecular  changes 
which  traverse  the  auditory  nerves  and  reach  the  cerebral 
ganglia. 

The  vibrations  of  the  luminiferous  ether  are  brought 
to  bear  upon  the  free  ends  of  two  large  bundles  of  nerve 
fibres,  termed  the  optic  nerves  (fig.  25,  on),  which  proceed 
directly  from  the  brain,  by  means  of  a  highly  complex  eye. 
This  is  an  apparatus,  which,  in  part,  sorts  out  the  rays  of 
light  into  as  many  very  small  pencils  as  there  are  separate 
endings  of  the  fibres  of  the  optic  nerve,  and,  in  part, 
serves  as  the  medium  by  which  the  luminous  vibrations 
are  converted  into  molecular  nerve  changes. 

The  free  extremity  of  the  eyestalk  presents  a  convex, 
soft,  and  transparent  surface,  limited  by  an  oval  contour. 
The  cuticle  in  this  region,  which  is  termed  the  cornea, 
(fig.  28,  a),  is,  in  fact,  somewrhat  thinner  and  less  dis- 
tinctly laminated  than  in  the  rest  of  the  ej^estalk,  and  it 
contains  no  calcareous  matter.  But  it  is  directly  con- 
tinuous with  the  rest  of  the  exoskeleton  of  the  eyestalk, 
to  which  it  stands  in  somewhat  the  same  relation  as  the 
soft  integument  of  an  articulation  does  to  the  adjacent 
hard  parts. 

The  cornea  is  divided  into  a  great  number  of  minute, 
usually  square  facets,  by  faint  lines,  which  cross  it  from  side 


THE    EYE   OF   THE   CRAYFISH. 


119 


to  side  nearly  at  right  angles  with  one  another.  A  longi- 
tudinal section  shows  that  both  the  horizontal  and  the 
vertical  contours  of  the  cornea  are  very  nearly  semicir- 
cular, and  that  the  lines  which  mark  off  the  facets  merely 
arise  from  a  slight  modification  of  its  substance  between 
the  facets.  The  outer  contour  of  each  facet  forms  part 


*P 


FIG.  28.—A*t<tcu*  ///»•/,/ fin*.  —A.  a  vertical  section  of  the  eye-stalk 
(  x  6)  ;  B,  a  small  portion  of  the  same,  showing  the  visual  ap- 
paratus more  highly  magnified  ;  a,  cornea  ;  b,  outer  dark  zone  ;  c, 
outer  white  zone  ;  d,  middle  dark  zone  ;  e,  inner  white  zone  ; 
/,  inner  dark  zone  ;  c>;  crystalline  cones  ;  g,  optic  ganglion  ;  op, 
optic  nerve  ;  xp.  striated  spindles. 


of  the  general  curvature  of  the  outer  face  of  the  cornea ; 
the  inner  contour  sometimes  exhibits  a  slight  deviation 


120      THE   PHYSIOLOGY   OF   THE   COMMON   CRAYFISH. 

from  the  general  curvature  of  the  inner  face,  but  usually 
nearly  coincides  with  it. 

When  a  longitudinal  or  a  transverse  section  is  taken 
through  the  whole  eyestalk,  the  optic  nerve  (fig.  28, 
A,  op)  is  seen  to  traverse  its  centre.  At  first  narrow 
and  cylindrical,  it  expands  towards  its  extremity  into 
a  sort  of  bulb  (B,  g),  the  outer  surface  of  which  is  curved 
in  correspondence  with  the  inner  surface  of  the  cornea. 
The  terminal  half  of  the  bulb  contains  a  great  quantity 
of  dark  colouring  matter  or  pigment,  and,  in  section, 
appears  as  what  may  be  termed  the  inner  dark  zone  (/). 
Outside  this,  and  in  connection  with  it,  follows  a  white 
line,  the  inner  white  zone  (e),  then  comes  a  middle  dark 
zone  (d)  ;  outside  this  an  outer  pale  band,  which  may 
be  called  the  outer  white  zone  (c),  and  between  this  and 
the  cornea  (a)  is  another  broad  band  of  dark  pigment,  the 
outer  dark  zone  (b). 

When  viewed  under  a  low  power,  by  reflected  light,  this 
outer  dark  zone  is  seen  to  be  traversed  by  nearly  parallel 
straight  lines,  each  of  which  starts  from  the  boundary 
between  two  facets,  and  can  be  followed  inwards  through 
the  outer  white  zone  to  the  middle  dark  zone.  Thus  the 
whole  substance  of  the  eye  between  the  outer  surface  of 
the  bulb  of  the  optic  nerve  and  the  inner  surface  of  the 
cornea  is  marked  out  into  as  many  segments  as  the 
cornea  has  facets ;  and  each  segment  has  the  form 
of  a  wedge  or  slender  pyramid,  the  base  of  which  is 
four-sided,  and  is  applied  against  the  inner  surface  of 


THE  VISUAL   PYRAMIDS.  121 

one  of  the  facets  of  the  cornea,  while  its  summit  lies  in 
the  middle  dark  zone.  Each  of  these  visual  pyramid* 
consists  of  an  axial  structure,  the  visual  rod,  invested  by 
a  sheath.  The  latter  extends  inwards  from  the  margin 
of  each  facet  of  the  cornea,  and  contains  pigment  in 
two  regions  of  its  length,  the  intermediate  space  being 
devoid  of  pigment.  As  the  position  of  the  pigmented 
regions  in  relation  to  the  length  of  the  pyramid  is  always 
the  same,  the  pigmented  regions  necessarily  take  the  form 
of  two  consecutive  zones  when  the  pjo-amids  are  in  their 
natural  position. 

The  visual  rod  consists  of  two  parts,  an  external 
wystalline  cone  (fig.  28,  B,  cr),  and  an  internal  striated 
spindle  (sp).  The  crystalline  cone  consists  of  a  trans- 
parent glassy-looking  substance,  which  may  be  made  to 
split  up  longitudinally  into  four  segments.  Its  inner  end 
narrows  into  a  filament  which  traverses  the  outer  white 
zone,  and,  in  the  middle  dark  zone,  thickens  into  a  four- 
sided  spindle-shaped  transparent  body,  which  appears 
transversely  striated.  The  inner  end  of  this  striated 
spindle  narrows  again,  and  becomes  continuous  with 
nerve  fibres  which  proceed  from  the  surface  of  the  optic 
bulb. 

The  exact  mode  of  connection  of  the  nerve-fibres  with 
the  visual  rods  is  not  certainly  made  out,  but  it  is  pro- 
bable that  there  is  direct  continuity  of  substance,  and  that 
each  rod  is  really  the  termination  of  a  nerve  fibre. 

Eyes  having  essentially  the  same  structure  as  that  of 


122      THE   PHYSIOLOGY    OF   THE   COMMON    CRAYFISH. 

the  crayfish  are  very  widely  met  with  among  Crustacea 
and  Insecta,  and  are  commonly  known  as  compound  eyes. 
In  many  of  these  animals,  in  fact,  when  the  cornea  is  re- 
moved, each  facet  is  found  to  act  as  a  separate  lens ;  and 
when  proper  arrangements  are  made,  as  many  distinct 
pictures  of  external  objects  are  found  behind  it  as  there 
are  facets.  Hence  the  notion  suggested  itself  that  each 
visual  pyramid  is  a  separate  eye,  similar  in  principle  of 
construction  to  the  human  eye,  and  forming  a  picture  of 
so  much  of  the  external  world  as  comes  within  the  range 
of  its  lens,  upon  a  retina  supposed  to  be  spread  out  on 
the  surface  of  the  crystalline  cone,  as  the  human  retina  is 
spread  over  the  surface  of  the  vitreous  humour. 

But,  in  the  first  place,  there  is  no  evidence,  nor  any 
probability,  that  there  is  anything  corresponding  to  a 
retina  on  the  outer  face  of  the  crystalline  cone ;  and 
secondly,  if  there  were,  it  is  incredible  that,  with  such  an 
arrangement  of  the  refractive  media  as  exists  in  the 
cornea  and  crystalline  cones,  rays  proceeding  from  points 
in  the  external  world  should  be  brought  to  a  focus  in  cor- 
respondingly related  points  of  the  surface  of  the  supposed 
retina.  But  without  this  no  picture  could  be  formed,  and 
no  distinct  vision  could  take  place.  It  is  very  probable, 
therefore,  that  the  visual  p3O*amids  do  not  play  the  part 
of  the  simple  eyes  of  the  Vertebrata,  and  the  only  alterna- 
tive appears  to  be  the  adoption  of  a  modification  of  the 
theory  of  mosaic  vision,  propounded  many  years  by 
Johannes  Miiller. 


THE  THEORY   OF  MOSAIC  VISION.  123 

Each  visual  p3Tamid.  isolated  from  its  fellows  by  its  coat 
of  pigment,  may  be  supposed,  in  fact,  to  play  the  part  of  a 
very  narrow  straight  tube,  with  blackened  walls,  one  end 
of  which  is  turned  towards  the  external  world,  while  the 
other  incloses  the  extremity  of  one  of  the  nerve  fibres.  The 
only  light  which  can  reach  the  latter,  under  these  circum- 
stances, is  such  as  proceeds  from  points  which  lie  in  the 


FIG.  29. — Diagram  showing-  the  course  of  rays  of  light  from  three 
points  ,r,  y,  r,  through  the  nine  visual  rods  (supposed  to  be  empty 
tubes)  A — I  of  a  compound  eye  ;  a—i,  the  nerve  fibres  connected 
with  the  visual  rods. 


direction  of  a  straight  line  represented  by  the  produced 
axis  of  the  tubes. 

Suppose  A — I  to  be  nine  such  tubes,  a — i  the  corre- 
sponding nerve  fibres,  and  x  y  z  three  points  from  which 
light  proceeds.  Then  it  will  be  obvious  that  the  only  light 


124      THE   PHYSIOLOGY  OF   THE  COMMON  CRAYFISH. 

from  x  which  will  excite  sensation,  will  be  the  ray  which 
traverses  B  and  reaches  the  nerve-fibre  b,  while  that  from 
y  will  affect  only  e,  and  that  from  x  only  h.  The  result, 
translated  into  sensation,  will  be  three  points  of  light  on  a 
dark  ground,  each  of  which  answers  to  one  of  the  luminous 
points,  and  indicates  its  direction  in  reference  to  the  eye 
and  its  angular  distance  from  the  other  two.* 

The  only  modification  needed  in  the  original  form  of 
the  theory  of  mosaic  vision,  is  the  supposition  that  part, 
or  the  whole,  of  the  visual  rod,  is  not  merely  a  passive 
transmitter  of  light  to  the  nerve-fibre,  but  is,  itself,  in 
someway  concerned  in  transmuting  the  mode  of  motion, 
light,  into  that  other  mode  of  motion  which  we  call 
nervous  energy.  The  visual  rod  is,  in  fact,  to  be  re- 
garded as  the  physiological  end  of  the  nerve,  and  the 
instrument  by  which  the  conversion  of  the  one  form  of 
motion  into  the  other  takes  place  ;  just  as  the  auditoiy 
hairs  are  instruments  by  which  the  sonorous  waves  are 
converted  into  molecular  movements  of  the  substance  of 
the  auditory  nerves. 

It  is  wonderfully  interesting  to  observe  that,  when  the 
so-called  compound  e}ye  is  interpreted  in  this  manner, 

*  Since  the  visual  rods  are  strongly  refracting-  solids,  and  not  empty 
tubes,  the  diagram  given  in  fig.  29  does  not  represent  the  true  course  of 
the  rays,  indicated  by  dotted  lines,  which  fall  obliquely  on  any  cornea 
of  a  crayfish's  eye.  Such  rays  will  be  more  or  less  bent  towards  the 
axis  of  the  visual  rod  of  that  cornea  ;  but  whether  they  reach  its  apex 
and  so  affect  the  nerve  or  not  will  depend  on  the  curvature  of  the  cornea ; 
its  refractive  index  and  that  of  the  crystalline  cone  ;  and  the  relation 
between  the  length  and  the  thickness  of  the  latter. 


DO   CRAYFISHES   HEAR  AND   SEE?  125 

the  apparent  wide  difference  between  it  and  the  verte- 
brate eye  gives  place  to  a  fundamental  resemblance.  The 
rods  and  cones  of  the  retina  of  the  vertebrate  eye  are 
extraordinarily  similar  in  their  form  and  their  relations 
to  the  fibres  of  the  optic  nerve,  to  the  visual  rods  of  the 
arthropod  eye.  And  the  morphological  discrepancy* 
which  is  at  first  so  striking,  and  which  arises  from  the 
fact  that  the  free  ends  of  the  visual  rods  are  turned 
towards  the  light,  while  those  of  the  rods  and  cones 
of  the  vertebrate  eye  are  turned  from  it,  becomes  a  confir- 
mation of  the  parallel  between  the  two  when  the  develop- 
ment of  the  vertebrate  eye  is  taken  into  account  For  it 
is  demonstrable  that  the  deep  surface  of  the  retina  in 
which  the  rods  and  cones  lie,  is  really  a  part  of  the  outer 
surface  of  the  body  turned  inwards,  in  the  course  of  the 
singular  developmental  changes  which  give  rise  to  the 
brain  and  the  eye  of  vertebrate  animals. 

Thus  the  crayfish  has,  at  any  rate,  two  of  the  higher 
sense  organs,  the  ear  and  the  eye,  which  we  possess  our- 
selves; and  it  may  seem  a  superfluous,  not  to  say  a 
frivolous,  question,  if  any  one  should  ask  whether  it  can 
hear  and  see. 

But,  in  truth,  the  inquiry,  if  properly  limited,  is  a  very 
pertinent  one.  That  the  crayfish  is  led  by  the  use  of  its 
eyes  and  ears  to  approach  some  objects  and  avoid  others, 
is  beyond  all  doubt ;  and,  in  this  sense,  most  indubit- 
ably it  can  both  hear  and  see.  But  ii  the  question 


126      THE   PHYSIOLOGY  OF   THE  COMMON   CRAYFISH. 

means,  do  luminous  vibrations  give  it  the  sensations  of 
light  and  darkness,  of  colour  and  form  and  distance,  which 
they  give  to  us  ?  and  do  sonorous  vibrations  produce  the 
feelings  of  noise  and  tone,  of  melody  and  of  harmony,  as 
in  us  ? — it  is  by  no  means  to  be  answered  hastily,  perhaps 
cannot  be  answered  at  all,  except  in  a  tentative,  probable 
way. 

The  phenomena  to  which  we  give  the  names  of  sound 
and  colour  are  not  physical '  things,  but  are  states  of  con- 
sciousness, dependent,  there  is  every  reason  to  believe, 
on  the  functional  activity  of  certain  parts  of  our  brains. 
Melody  and  harmony  are  names  for  states  of  conscious- 
ness which  arise  when  at  least  two  sensations  of  sound 
have  been  produced.  All  these  are  manufactured  arti- 
cles, products  of  the  human  brain  ;  and  it  would  be 
exceedingly  hazardous  to  affirm  that  organs  capable  of 
giving  rise  to  the  same  products  exist  in  the  vastly 
simpler  nervous  s}Tstem  of  the  crustacean.  It  would  be 
the  height  of  absurdity  to  expect  from  a  meat-jack  the 
sort  of  work  which  is  performed  by  a  Jacquard  loom  ;  and 
it  appears  to  me  to  be  little  less  preposterous  to  look  for 
the  production  of  anything  analogous  to  the  more  subtle 
phenomena  of  the  human  mind  in  something  so  minute 
and  rude  in  comparison  to  the  human  brain,  as  the 
insignificant  cerebral  ganglia  of  the  crayfish. 

At  the  most,  one  may  be  justified  in  supposing  the 
existence  of  something  approaching  dull  feeling  in  our- 
selves ;  and,  to  return  to  the  problem  stated  in  the  begin- 


THE   MORTALITY  OF  CRAYFISHES.  127 

ning  of  this  chapter,  so  far  as  such  obscure  consciousness 
accompanies  the  molecular  changes  of  its  nervous  sub- 
stance, it  will  be  right  to  speak  of  the  mind  of  a  crayfish. 
But  it  will  be  obvious  that  it  is  merely  putting  the  cart 
before  the  horse,  to  speak  of  such  a  mind  as  a  factor 
in  the  work  done  by  the  organism,  when  it  is  merely  a 
dim  symbol  of  a  part  of  such  work  in  the  doing. 

Whether  the  crayfish  possesses  consciousness  or  not, 
however,  does  not  affect  the  question  of  its  being  an 
engine,  the  actions  of  which  at  any  moment  depend,  on 
the  one  hand,  upon  the  series  of  molecular  changes  excited, 
either  by  internal  or  by  external  causes,  in  its  neuro- 
muscular  machinery ;  and,  on  the  other,  upon  the  dispo- 
sition and  the  properties  of  the  parts  of  that  machinery. 
And  such  a  self-adjusting  machine,  containing  the  im- 
mediate conditions  of  its  action  within  itself,  is  what  is 
properly  understood  by  an  automaton. 

Crayfishes,  as  we  have  seen,  may  attain  a  considerable 
age ;  and  there  is  no  means  of  knowing  how  long  they 
might  live,  if  protected  from  the  innumerable  destructive 
influences  to  which  they  are  at  all  ages  liable. 

It  is  a  widely  received  notion  that  the  energies  of  living 
matter  have  a  natural  tendency  to  decline,  and  finally 
disappear ;  and  that  the  death  of  the  body,  as  a  whole, 
is  the  necessary  correlate  of  its  life.  That  all  living 
things  sooner  or  later  perish  needs  no  demonstration, 
but  it  would  be  difficult  to  find  satisfactory  grounds 


128      THE  PHYSIOLOGY  OF  THE  COMMON   CRAYFISH. 

for  the  belief  that  they  must  needs  do  so.  The  analogy  of 
a  machine  that,  sooner  or  later,  must  be  brought  to  a 
standstill  by  the  wear  and  tear  of  its  parts,  does  not 
hold,  inasmuch  as  the  animal  mechanism  is  continually 
renewed  and  repaired  ;  and,  though  it  is  true  that  indi- 
vidual components  of  the  body  are  constantly  dying,  yet 
their  places  are  taken  by  vigorous  successors.  A  city 
remains,  notwithstanding  the  constant  death-rate  of  its 
inhabitants ;  and  such  an  organism  as  a  crayfish  is  only 
a  corporate  unity,  made  up  of  innumerable  partially 
independent  individualities. 

Whatever  might  be  the  longevity  of  crayfishes  under 
imaginable  perfect  conditions,  the  fact  that,  notwithstand- 
ing the  great  number  of  eggs  they  produce,  their  number 
remains  pretty  much  the  same  in  a  given  district,  if 
we  take  the  average  of  a  period  of  years,  shows  that 
about  as  many  die  as  are  born ;  and  that,  without  the 
process  of  reproduction,  the  species  would  soon  come  to 
an  end. 

There  are  many  examples  among  members  of  the  group 
of  Crustacea  to  which  the  crayfish  belongs,  of  animals  which 
produce  young  from  internally  developed  germs,  as  some 
plants  throw  off  bulbs  which  are  capable  of  reproducing 
the  parent  stock ;  such  is  the  case,  for  example,  with  the 
common  water  flea  (Daphnia).  But  nothing  of  this  kind 
has  been  observed  in  the  crayfish ;  in  which,  as  in  the 
higher  animals,  the  reproduction  of  the  species  is  de- 
pendent upon  the  combination  of  two  kinds  of  living 


THE   OVARY  AND  THE  TESTIS.  129 

matter,  which  are  developed  in  different  individuals, 
termed  males  and  females. 

These  two  kinds  of  living  matter  are  ova  and  sperma- 
tozoa, and  they  are  developed  in  special  organs,  the  ovary 
and  the  testis.  The  ovary  is  lodged  in  the  female  ;  the 
testis,  in  the  male. 

The  ocary  (fig.  30,  ov)  is  a  body  of  a  trefoil  form, 
which  is  situated  immediately  beneath,  or  in  front  of, 
the  heart,  between  the  floor  of  the  pericaHinl  sinus  and 
the  alimentary  canal.  From  the  ventral  lace  of  this 


Oft' 


FIG.  30.  —  AshicHs  jlurintil'ts. — The  female  reproductive  organs  (  x  2); 
oi',  ovary  ;  od,  oviduct ;  od',  aperture  of  oviduct. 

organ  two  short  and  wide  canals,  the  oviducts  (od),  lead 
down  to  the  bases  of  the  second  pair  of  walking  limbs, 
and  terminate  in  the  apertures  (od')  already  noticed 
there. 

The  testis  (fig.  31,    t)  is  somewhat  similar  in  form  to 
the   ovary,   but,  the  three  divisions  are  much  narrower 
10 


130  THE   PHYSIOLOGY   OF  THE   COMMON   CRAYFISH. 

and  more  elongated  :  the  hinder  median  division  lies 
under  the  heart ;  the  anterior  divisions  are  situated 
between  the  heart  behind,  and  the  stomach  and  the  liver 
in  front  (figs.  5  and  12,  i).  From  the  point  at  which  the 


FlG.  31. — Astacus  flnviatilis. — The  male  reproductive  organs  ( x  2)  ; 
t ,  testis  ;  vd,  vas  deferens  ;  vd',  aperture  of  vas  deferens. 

three  divisions  join,  proceed  two  ducts,  which  are  termed 
the  vasa  defer  entia  (fig.  31,  vd).  These  are  very  narrow, 
long,  and  make  many  coils  before  they  reach  the  apertures 
upon  the  bases  of  the  hindermost  pair  of  walking  limbs,  by 
which  they  open  externally  (fig.  31,  rd',  nnd  fig.  35,  r<7). 
Both  the  ovary  and  the  testis  are  very  much  larger 


THE  OVARY  AND  THE  EGGS. 


is: 


during  the  breeding  season  than  at  other  times ;  the  large 
brownish-yellow  eggs  become  conspicuous  in  the  ovary, 


FIG.  32.  —  AstacusJluvuitirts.—A,3L  two-thirds  grown  egg  contained  in 
its  ovisac  ( x  50)  ;  B,  an  egg  removed  from  the  ovisac  (  x  10)  ; 
C,  a  portion  of  the  wall  of  an  ovisac  with  the  adjacent  portion  of 
the  contained  egg,  highly  magnified  ;  ep.  epithelium  of  ovisac  ; 
gs,  germinal  spots  ;  gr,  germinal  vesicle  ;  ;w,  membrana  propria  ; 
r,  vitellus  :  r/w,  vitelline  membrane  ;  /r,  stalk  of  ovisac. 

and   the   testis    assumes   a   milk-white    colour,    at   this 
period. 

The  walls  of  the  ovary  are  lined  internally  by  a  layer  of 


132 


THE   PHYSIOLOGY   OF  THE   COMMON   CRAYFISH. 


nucleated  cells,  separated  from  the  cavity  of  the  organ  by 
a  delicate  structureless  membrane.  The  growth  of  these 
cells  gives  rise  to  papillary  elevations  which  project  into 
the  cavity  of  the  ovary,  and  eventually  become  globular 


B 


FlG.  33. — AttaeiM  fluviahli*. — A,  a  lobule  of  the  testis,  showing-  a,  acini, 
springing  from  I,  the  ultimate  termination  of  a  duct  (x  50).  B, 
spermatic  cells  ;  a,  with  an  ordinary  globular  nucleus  n  ;  I,  with  a 
spindle-shaped  nucleus  ;  c,  with  two  similar  nuclei ;  and  d,  with 
a  nucleus  undergoing  division  (  x  600). 

bodies  attached  by  short  stalks,  and  invested  by  the  struc- 
tureless membrane  as  a  membrana  propria  (fig.  32,  ?;i). 
These  are  the  ovisacs.  In  the  mass  of  cells  which  be- 
comes the  ovisac,  one  rapidly  increases  in  size  and 
occupies  the  centre  of  the  ovisac,  while  the  others 


THE  OVA  AND  THE  SPERMATOZOA.  133 

surround  it  as  a  peripheral  coat  (ep.).  This  central  cell 
is  the  ornm.  Its  nucleus  enlarges,  and  becomes  what  is 
called  the  germinal  vesicle  (g.v.).  At  the  same  time 
numerous  small  corpuscles,  flattened  externally  and 
convex  internally,  appear  in  it  and  are  the  germinal 
spots  (g.s.).  The  protoplasm  of  the  cell,  as  it  enlarges, 
becomes  granular  and  opaque,  assumes  a  deep  brownish- 
yellow  colour,  and  is  thus  converted  into  the  yelk  or 
vitellus  (r.).  As  the  egg  grows,  a  structureless  vitelline 
membrane  is  formed  between  the  vitellus  and  the  cells 
which  line  the  ovisac,  and  incloses  the  egg,  as  in  a 
bag.  Finally,  the  ovisac  bursts,  and  the  egg,  falling 
into  the  cavity  of  the  ovary,  makes  its  way  down  the 
oviduct,  and  sooner  or  later  passes  out  by  its  aperture. 
When  they  leave  the  oviduct,  the  ova  are  invested  by 
a  viscous,  transparent  substance,  which  attaches  them 
to  the  swimmerets  of  the  female,  and  then  sets ;  thus 
each  egg,  inclosed  in  a  tough  case,  is  firmly  suspended 
by  a  stalk,  which,  on  the  one  side,  is  continued  into  the 
substance  of  the  case,  while,  on  the  other,  it  is  fixed  to 
the  swhnrneret.  The  swimmerets  are  kept  constantly  in 
motion,  so  that  the  eggs  are  well  supplied  with  aerated 
water. 

The  testis  consists  of  an  immense  number  of  minute 
spheroidal  vesicles  (fig.  33,  A,  a),  attached  like  grapes  to 
the  ends  of  short  stalks  (6),  formed  by  the  ultimate 
ramifications  of  the  vasa  deferentia.  The  vesicles  may, 
in  fact,  be  regarded  as  dilatations  of  the  ends  and  sides 


Fro.  34. — Astacus  fluviatilis. — A — D,  different  stages  in  the  development  of  a  sperma- 
tozoon from  a  seminal  cell ;  E,  a  mature  spermatozoon  seen  from  the  side  ;  F,  the 
same  viewed  en  face  (all  x  850) ;  G,  a  diagrammatic  vertical  section  of  the  same. 


THE   PROCESS   OF   FERTILIZATION.  J35 

of  the  finest  branches  of  the  ducts  of  the  testis.  The 
cavity  of  each  vesicle  is  filled  by  the  large  nucleated  cells 
which  line  its  walls  (fig.  33,  B),  and,  as  the  breeding 
season  approaches,  these  cells  multiply  by  division. 
Finally,  they  undergo  some  very  singular  changes  of 
form  and  internal  structure  (fig.  34,  A — D),  each  becom- 
ing converted  into  a  flattened  spheroidal  body,  about 
Yy^gth  of  an  inch  in  diameter,  provided  with  a  number 
of  slender  curved  rays,  which  stand  out  from  its  sides 
(fig.  34,  E — G).  These  are  the  spermatozoa. 

The  spermatozoa  accumulate  in  the  testicular  vesicles, 
and  give  rise  to  a  milky-looking  substance,  which  traverses 
the  smaller  ducts,  and  eventually  fills  the  vasa  deferentia. 
This  substance,  however,  consists,  in  addition  to  the 
spermatozoa,  of  a  viscid  material,  secreted  by  the  walls 
of  the  vasa  deferentia,  which  envelopes  the  spermatozoa, 
and  gives  the  secretion  of  the  testis  the  form  and  the 
consistency  of  threads  of  vermicelli. 

The  ripening  and  detachment  of  both  the  ova  and 
the  spermatozoa  take  place  immediately  after  the  com- 
pletion of  ecdysis  in  the  early  autumn;  and  at  this 
time,  which  is  the  breeding  season,  the  males  seek 
the  females  with  great  avidity,  in  order  to  deposit  the 
fertilizing  matter  contained  in  the  vasa  deferentia  on  the 
sterna  of  their  hinder  thoracic  and  anterior  abdominal 
somites.  There  it  adheres  as  a  whitish,  chalky-looking 
mass  ;  but  the  manner  in  which  the  contained  sperma- 
tozoa reach  and  enter  the  ova  is  unknown.  The  analogy 


136 


THE   PHYSIOLOGY   OF    THE   COMMON    CRAYFISH. 


of  what  occurs  in  other  animals,  however,  leaves  no  doubt 
that  an  actual  mixture  of  the  male  and  female  ele- 
ments takes  place  and  constitutes  the  essential  part  of 
the  process  of  impregnation. 

Ova  to  which  spermatozoa  have  had  no  access, 
give  rise  to  no  progeny ;  but,  in  the  impregnated  ovum, 
the  young  crayfish  takes  its  origin  in  a  manner  to  be 
described  below,  when  the  question  of  development  is 
dealt  with. 


FlG.  tt.  —  Astacus  fluviatttis. — The  last  thoracic  sternum,  seen  from 
behind,  with  the  proximal  ends  of  the  appendages,  A,  in  the  male, 
B,  in  the  female,  (  x  3).  am,  articular  membrane  ;  cxp,  coxopo- 
dite  ;  st  XIV \  last  thoracic  sternum  ;  vd,  aperture  of  vas  deferens» 


CHAPTEE  IV. 

THE  MORPHOLOGY  OF  THE    COMMON    CRAYFISH:    THE  STRUC- 
TURE    AND    THE    DEVELOPMENT    OF    THE    INDIVIDUAL. 

IN  the  two  preceding  chapters  the  crayfish  has  been 
studied  from  the  point  of  view  of  the  physiologist,  who, 
regarding  an  animal  as  a  mechanism,  endeavours  to  dis- 
cover how  it  does  that  which  it  does.  And,  practically,  this 
way  of  looking  at  the  matter  is  the  same  as  that  of  the 
teleologist.  For,  if  all  that  we  know  concerning  the  pur- 
pose of  a  mechanism  is  derived  from  observation  of  the 
manner  in  which  it  acts,  it  is  all  one,  whether  we  say 
that  the  properties  and  the  connexions  of  its  parts 
account  for  its  actions,  or  that  its  structure  is  adapted 
to  the  performance  of  those  actions. 

Hence  it  necessarily  follows  that  physiological  pheno- 
mena can  be  expressed  in  the  language  of  teleology. 
On  the  assumption  that  the  preservation  of  the  indi- 
vidual, and  the  continuance  of  the  species,  are  the 
final  causes  of  the  organization  of  an  animal,  the  exist- 
ence of  that  organization  is,  in  a  certain  sense,  explained, 
when  it  is  shown  that  it  is  fitted  for  the  attainment  of 
those  ends  ;  although,  perhaps,  the  importance  of  de- 


138       THE  MORPHOLOGY  OF  THE   COMMON   CRAYFISH. 

monstrating  the  proposition  that  a  thing  is  fitted  to  do 
that  which  it  does,  is  not  very  great. 

But  whatever  may  he  the  value  of  Ideological  ex- 
planations, there  is  a  large  series  of  facts,  which  have  as 
yet  heen  passed  over,  or  touched  only  incidentally,  of 
which  they  take  no  account.  These  constitute  the  sub- 
ject matter  of  Morphology,  which  is  related  to  physiology 
much  as,  in  the  not-living  world,  crystallography  is 
related  to  the  study  of  the  chemical  and  physical  pro- 
perties of  minerals. 

Carbonate  of  lime,  for  example,  is  a  definite  compound 
of  calcium,  carbon,  and  oxygen,  and  it  has  a  great  variety 
of  physical  and  chemical  properties.  But  it  may  be 
studied  under  another  aspect,  as  a  substance  capable  of 
assuming  crystalline  forms,  which,  though  extraordinarily 
various,  may  all  be  reduced  to  certain  geometrical  types. 
It  is  the  business  of  the  crystallographer  to  work  out 
the  relations  of  these  forms ;  and,  in  so  doing,  he  takes  no 
note  of  the  other  properties  of  carbonate  of  lime. 

In  like  manner,  the  morphologist  directs  his  attention 
to  the  relations  of  form  between  different  parts  of  the 
same  animal,  and  between  different  animals;  and  these 
relations  would  be  unchanged  if  animals  were  mere 
dead  matter,  devoid  of  all  physiological  properties — a 
kind  of  mineral  capable  of  a  peculiar  mode  of  growth. 

A  familiar  exemplification  of  the  difference  between 
teleology  and  morphology  may  be  found  in  such  works 
of  human  art  as  houses. 


TELEOLOGY  AND  MOKPHOLOGT.  139 

A  house  is  certainly,  to  a  great  extent,  an  illustration 
of  adaptation  to  purpose,  and  its  structure  is,  to  that 
extent,  explicable  by  teleological  reasonings.  The  roof 
and  the  walls  are  intended  to  keep  out  the  weather;  the 
foundation  is  meant  to  afford  support  and  to  exclude 
damp ;  one  room  is  contrived  for  the  purpose  of  a 
kitchen;  another  for  that  of  a  coal-cellar;  a  third  for 
that  of  a  dining-room;  others  are  constructed  to  serve  as 
sleeping  rooms,  and  so  on ;  doors,  chimne}'s,  windows, 
drains,  are  all  more  or  less  elaborate  contrivances  directed 
towards  one  end,  the  comfort  and  health  of  the  dwellers 
in  the  house.  What  is  sometimes  called  sanitary  architec- 
ture, now-a-days,  is  based  upon  considerations  of  house 
teleology.  But  though  all  houses  are,  to  begin  with  and 
essentially,  means  adapted  to  the  ends  of  shelter  and 
comfort,  they  may  be,  and  too  often  are,  dealt  with  from 
a  point  of  view,  in  which  adaptation  to  purpose  is  largely 
disregarded,  and  the  chief  attention  of  the  architect  is 
given  to  the  form  of  the  house.  A  house  may  be  built  in 
the  Gothic,  the  Italian,  or  the  Queen  Anne  style  ;  and  a 
house  in  any  one  of  these  styles  of  architecture  may  be 
just  as  convenient  or  inconvenient,  just  as  well  or  as  ill 
adapted  to  the  wants  of  the  resident  therein,  as  any  of 
the  others.  Yet  the  three  are  exceedingly  different. 

To  apply  all  this  to  the  crayfish.  It  is,  in  a  sense, 
a  house  with  a  great  variety  of  rooms  and  offices,  in 
which  the  work  of  the  indwelling  life  in  feeding,  breath- 
ing, moving,  and  reproducing  itself,  is  done.  But  the 


140       THE  MORPHOLOGY  OF   THE  COMMON   CRAYFISH. 

same  may  be  said  of  the  crayfish's  neighbours,  the  perch 
and  the  water-snail ;  and  they  do  all  these  things  neither 
better  nor  worse,  in  relation  to  the  conditions  of  their 
existence,  than  the  crayfish  does.  Yet  the  most  cursory 
inspection  is  sufficient  to  show  that  the  "  styles  of  archi- 
tecture "  of  the  three  are  even  more  widely  different  than 
are  those  of  the  Gothic,  Italian,  and  Queen  Anne  houses. 
That  which  Architecture,  as  an  art  conversant  with 
pure  form,  is  to  buildings,  Morphology,  as  a  science 
conversant  with  pure  form,  is  to  animals  and  plants. 
And  we  may  now  proceed  to  occupy  ourselves  exclusively 
with  the  morphological  aspect  of  the  crayfish. 

As  I  have  already  mentioned,  when  dealing  with  the 
physiology  of  the  crayfish,  the  entire  body  of  the  animal, 
when  reduced  to  its  simplest  morphological  expression, 
may  be  represented  as  a  cylinder,  closed  at  each  end,  ex- 
cept so  far  as  it  is  perforated  by  the  alimentary  aper- 
tures (fig.  6) ;  or  we  may  say  that  it  is  a  tube,  inclosing 
another  tube,  the  edges  of  the  two  being  continuous  at 
their  extremities.  The  outer  tube  has  a  chitinous  outer 
coat  or  cuticle,  which  is  continued  on  to  the  inner  face 
of  the  inner  tube.  Neglecting  this  for  the  present,  the 
outermost  part  of  the  wall  of  the  outer  tube,  which 
answers  to  the  epidermis  of  the  higher  animals,  and  the 
innermost  part  of  the  wall  of  the  inner  tube,  which  is 
an  epithelium,  are  formed  by  a  layer  of  nucleated  cells. 
A  continuous  layer  of  cells,  therefore,  is  everywhere  to 


ENDODERM,   MESODERM,  AND   ECTODERM.  141 

be  found  on  both  the  external  and  the  internal  free  sur- 
faces of  the  body.  So  far  as  these  cells  belong  to  the 
proper  external  wall  of  the  body,  they  constitute  the 
ectoderm,  and  so  far  as  they  belong  to  its  proper  internal 
wall,  they  compose  the  endoderm.  Between  these  two 
layers  of  nucleated  cells  lie  all  the  other  parts  of  the 
body,  composed  of  connective  tissue,  muscles,  vessels, 
and  nerves ;  and  all  these  (with  the  exception  of  the 
ganglionic  chain,  which  we  shall  see  properly  belongs  to 
the  ectoderm)  may  be  regarded  as  a  single  thick  stratum, 
which,  as  it  lies  between  the  ectoderm  and  the  endoderm, 
is  called  the  mesoderm. 

If  the  intestine  were  closed  posteriorly  instead  of 
opening  by  the  vent,  the  crayfish  would  virtually  be  an 
elongated  sac,  with  one  opening,  the  mouth,  affording  an 
entrance  into  the  alimentary  cavity:  and,  round  this 
cavity,  the  three  layers  just  referred  to  —  endoderm, 
mesoderm,  and  ectoderm — would  be  disposed  concen- 
trically. 

We  have  seen  that  the  body  of  the  crayfish  thus  com- 
posed is  obviously  separable  into  three  regions — the 
cephalon  or  head,  the  thorax,  and  the  abdomen.  The 
latter  is  at  once  distinguished  by  the  size  and  the 
mobility  of  its  segments  :  while  the  thoracic  region  ia 
marked  off  from  that  of  the  head,  outwardly,  only  by  the 
nervical  groove.  But,  when  the  carapace  is  removed, 
the  lateral  depression  already  mentioned,  in  which  the 


142       THE   MORPHOLOGY    OF   THE    COMMON    CRAYFISH. 

scaphognatliite  lies,  clearly  indicates  the  natural  boundary 
between  the  head  and  the  thorax.  It  has  further  been 
observed  that  there  are,  in  all,  twenty  pairs  of  ap- 
pendages, the  six  hindermost  of  which  are  attached  to 
the  abdomen.  If  the  other  fourteen  pairs  are  carefully 
removed,  it  will  be  found  that  the  six  anterior  belong  to 
the  head,  and  the  eight  posterior  to  the  thorax. 

The  abdominal  region  may  now  be  studied  in  further 
detail.  Each  of  its  seven  movable  segments,  except  the 
telson,  represents  a  sort  of  morphological  unit,  the  repe- 
tition of  which  makes  up  the  whole  fabric  of  the  body. 

If  the  abdomen  is  divided  transversely  between  the 


t?lG.  36.—  Astaciis  flvriatilix.—  A  transverse  section  through  the  nine- 
teenth (fifth  abdominal)  somite  (  x  2).  e.m.,  extensor  muscles  ;  f.m., 
flexor  muscles  ;  gn.  If,  the  fifth  abdominal  ganglion  ;  h.g.,  hind-gut ; 
La. a.,  inferior  abdominal  artery  ;  s.afi,  superior  abdominal  artery  ; 
pi.  XIX,  pleura  of  the  somite  ;  st.  XIX,  its  sternum  ;  t.  XIX,  ita 
tergum  ;  ep.  XIX,  its  epimera  ;  19,  its  appendages. 


SOMITES  AND  APPENDAGES.  143 

fourth  and  fifth,  and  the  fifth  and  sixth  segments,  the  fifth 
will  be  isolated,  and  can  be  studied  apart.  It  constitutes 
what  is  called  a  metamere ;  in  which  are  distinguishable  a 
central  part  termed  the  somite,  and  two  appendages 
(fig.  36). 

In  the  exoskeleton  of  the  somites  of  the  abdomen 
several  regions  have  already  been  distinguished;  and 
although  they  constitute  one  continuous  whole,  it  will 
be  convenient  to  speak  of  the  sternum  (fig.  36,  st.  XIX), 
the  tergum  (t.  XIX),  and,  the  pleura  (pi.  XIX),  as  if  they 
were  separate  parts,  and  to  distinguish  that  portion  of 
the  sternal  region,  which  lies  between  the  articulation 
of  the  appendage  and  the  pleuron,  on  each  side,  as  the 
epimeron  (ep.  XIX).  Adopting  this  nomenclature,  it  may 
be  said  of  the  fifth  somite  of  the  abdomen,  that  it 
consists  of  a  segment  of  the  exoskeleton,  divisible  into 
tergum,  pleura,  epimera,  and  sternum,  with  which  two 
appendages  are  articulated;  that  it  contains  a  double 
ganglion  (gn.  12},  a  section  of  the  flexor  (fm)  and  extensor 
(em)  muscles,  and  of  the  alimentary  (hg)  and  vascular 
(s.a.a,  i.a.a)  systems. 

The  appendage  (fig.  36,  19),  which  is  attached  to  an 
articular  cavity  situated  between  the  sternum  and  the 
epimeron,  is  seen  to  consist  of  a  stalk  or  stem,  which  is 
made  up  of  a  very  short  basal  joint,  the  coxopodite  (fig.  37, 
D  and  E,  cx.p),  followed  by  a  long  cylindrical  second 
joint,  the  basipodite  (b.p),  and  receives  the  name  of  pro- 
topodite.  At  its  free  end,  it  bears  two  flattened  narrow 


cxp 


FIG.  37. — Astacus  fl-iiviatilis.— Appendages  of  the  left  side  of  the  abdo- 
men (  x  3).  A.  the  posterior  face  of  the  first  appendage  of  the  male  ; 
B,  the  same  of  the  female  ;  C,  posterior,  and  C',  anterior  faces  of  the 
second  appendage  of  the  male  ;  D,  the  third  appendage  of  the  male  : 
E,  the  same  of  the  female  -,  F,  the  sixth  appendage,  a,  the  rolled 
plate  of  the  endopodite  ;  ft,  the  jointed  extremity  of  the  same  ;  bp., 
basipodite  ;  cx.p»  coxopodite  ;  en.p.,  endopodite  ;  ex. p.,  exopodite. 


SOMITES  AND   APPENDAGES.  145 

plates,  of  which  one  is  attached  to  the  inner  side  of  the 
extremity  of  the  protopodite,  and  is  called  the  endopodite 
(en.p),  while  the  other  is  fixed  a  little  higher  up  to  the 
outer  side  of  that  extremity,  and  is  the  exopodite  (ex.p). 
The  exopodite  is  shorter  than  the  endopodite.  The 
endopodite  is  broad  and  is  undivided  for  about  half  its 
length,  from  the  attached  end ;  the  other  half  is  narrower, 
and  is  divided  into  a  number  of  small  segments,  which, 
however,  are  not  united  by  definite  articulations,  but  are 
merely  marked  off  from  one  another  by  slight  constric- 
tions of  the  exoskeleton.  The  exopodite  has  a  similar 
structure,  but  its  undivided  portion  is  shorter  and  nar- 
rower. The  edges  of  both  the  exopodite  and  the  endo- 
podite are  fringed  with  long  setaB. 

In  the  female  crayfish,  the  appendages  of  this  and  of 
the  fourth  and  third  somites  are  larger  than  in  the  male 
(compare  D  and  E,  fig.  37). 

The  fourth  and  fifth  somites,  with  their  appendages, 
may  be  described  in  the  same  terms  as  the  third,  and 
in  the  sixth  there  is  no  difficulty  in  recognising  the 
corresponding  parts  of  the  somite ;  but  the  appendages 
(fig.  37,  F)t  which  constitute  the  lateral  portions  of 
the  caudal  fin,  at  first  sight  appear  very  different.  In 
their  size,  no  less  than  in  their  appearance,  they  depart 
widely  from  the  appendages  of  the  preceding  somites. 
Nevertheless,  each  will  be  found  to  consist  of  a  basal 
stalk,  answering  to  the  protopodite  (cx.p),  which  how- 
ever is  very  broad  and  thick,  and  is  not  divided  into  two 
11 


146        THE   MORPHOLOGY   OF   THE   COMMON   CRAYFISH. 

joints  ;  and  of  two  terminal  oval  plates,  which  represent 
the  endopodite  (en.p)  and  the  exopodite  (ex.p).  The 
latter  is  divided  by  a  transverse  suture  into  two  pieces ; 
and  the  edge  of  the  larger  or  basal  moiety  is  beset  with 
short  spines,  of  which  two,  at  the  outer  end  of  the  series, 
are  larger  than  the  rest. 

The  second  somite  is  longer  than  the  first  (fig.  1) ;  it 
has  ver}'  broad  pleura,  while  those  of  the  first  somite  are 
small  and  hidden  by  the  overlapping  front  margins  of  the 
pleura  of  the  second  somite. 

In  the  female,  the  appendages  of  the  second  somite  of 
the  abdomen  are  similar  to  those  of  the  third,  fourth,  and 
fifth  somites ;  but  in  those  of  the  first  somite  (fig.  37,  .B), 
there  is  a  considerable  variation.  Sometimes,  in  fact, 
the  appendages  of  this  somite  are  altogether  wanting ; 
sometimes  one  is  present,  and  not  the  other ;  and 
sometimes  both  are  found.  But,  when  they  exist,  these 
appendages  are  always  small ;  and  the  protopodite  is 
followed  by  only  one  imperfectly  jointed  filament,  which 
appears  to  represent  the  endopodite  of  the  other  ap- 
pendages. 

In  the  male,  the  appendages  of  the  first  and  second 
somites  of  the  abdomen  are  not  only  of  relatively  large 
size,  but  they  are  widely  different  from  the  rest,  those  of 
the  first  somite  departing  from  the  general  type  further 
than  those  of  the  second.  In  the  latter  (C,  C ')  there  is 
a  protopodite  (cx.p,  bp)  with  the  ordinary  structure,  and 
it  is  followed  by  an  endopodite  (en.p)  and  an  exopodite 


SOMITES  AND  APPENDAGES.  147 

(fx.p) ;  but  the  former  is  singularly  modified.  The  un- 
divided basal  part  is  large,  and  is  produced  on  the 
inner  »ide  into  a  lamella  (a),  which  extends  slightly 
beyond  the  end  of  the  terminal  jointed  portion  (b).  The 
inner  half  of  this  lamella  is  rolled  upon  itself,  in  such  a 
manner  as  to  give  rise  to  a  hollow  cone,  something  like 
an  extinguisher  (C't  a). 

The  appendage  of  the  first  somite  (A)  is  an  unjoin  ted 
styliform  body,  which  appears  to  represent  the  proto- 
podite,  together  with  the  basal  part  and  the  inner  pro- 
longation of  the  endopodite  of  the  preceding  appendage. 
The  terminal  half  of  the  appendage  is  really  a  broad 
plate,  slightly  bifid  at  the  summit,  but  the  sides  of  the 
plate  are  rolled  in,  in  such  a  manner  that  the  anterior 
half  bends  round  and  partially  incloses  the  posterior  half. 
They  thus  give  rise  to  a  canal,  which  is  open  at  each  end, 
and  only  partially  closed  behind. 

These  two  pairs  of  curiously  modified  appendages  are 
ordinarily  turned  forwards  and  applied  against  the  sterna 
of  the  posterior  part  of  tha  thorax,  in  the  interval  be- 
tween the  bases  of  the  hinder  thoracic  limbs  (see  fig.  3, 
A).  They  serve  as  conduits  by  which  the  spermatic 
matter  of  the  male  is  conveyed  from  the  openings  of  the 
ducts  of  the  testes  to  its  destination. 

If  we  confine  our  attention  to  the  third,  fourth,  and 
fifth  metameres  of  the  abdomen  of  the  crayfish,  it  is 
obvious  that  the  several  somites  and  their  appendages, 
and  the  various  regions  or  parts  into  which  they  are 


148       THE  MORPHOLOGY  OF  THE   COMMON   CRAYFISH. 

divisible,  correspond  with  one  another,  not  only  in  form, 
but  in  their  relations  to  the  general  plan  of  the  whole 
abdomen.  Or,  in  other  words,  a  diagrammatic  plan  of 
one  somite  will  serve  for  all  the  three  somites,  with 
insignificant  variations  in  detail.  The  assertion  that 
these  somites  are  constructed  upon  the  same  plan,  in- 
volves no  more  hypothesis  than  the  statement  of  an 
architect,  that  three  houses  are  built  upon  the  same  plan, 
though  the  facades  and  the  internal  decorations  may 
differ  more  or  less. 

In  the  language  of  morphology,  such  conformity  in  the 
plan  of  organisation  is  termed  homology.  Hence,  tlu 
several  metameres  in  question  and  their  appendages,  are 
homologous  with  one  another ;  while  the  regions  of  the 
somites,  and  the  'parts  of  their  appendages,  are  also 
homologues. 

When  the  comparison  is  extended  to  the  sixth  meta- 
mere,  the  homology  of  the  different  parts  with  those  of  the 
other  metameres,  is  undeniable,  notwithstanding  the  great 
differences  which  they  present.  To  recur  to  a  previous 
comparison,  the  ground  plan  of  the  building  is  the  same, 
though  the  proportions  are  varied.  So  with  regard  to 
the  first  and  second  metameres.  In  the  sec  end  pair 
of  appendages  of  the  male,  the  difference  from  the 
ordinary  type  of  appendage  is  comparable  to  that  pro- 
duced by  adding  a  portico  or  a  turret  to  the  building ; 
while,  in  the  first  pair  of  appendages  of  the  female, 
it  is  as  if  one  wing  of  the  edifice  were  left  unbuilt; 


HOMOLOGY  AND  HOMOLOGUES.         149 

and,  in  those  of  the  male,  as  if  all  the  rooms  were  run 
into  one. 

It  is  further  to  be  remarked,  that,  just  as  of  a  row  of 
houses  built  upon  the  same  plan,  one  ma}r  be  arranged  so  as 
to  serve  as  a  dwelling-house,  another  as  a  warehouse,  and 
another  as  a  lecture  hall,  so  the  homologous  appendages 
of  the  crayfish  are  made  to  subserve  various  functions. 
And  as  the  fitness  of  the  dwelling-house,  the  warehouse, 
and  the  lecture-hall  for  their  several  purposes  would  not 
in  the  least  help  us  to  understand  why  they  should  all  be 
built  upon  the  same  general  plan ;  so,  the  adaptation  of 
the  appendages  of  the  abdomen  of  the  crayfish  to  the  dis- 
charge of  their  several  functions  does  not  explain  why 
those  parts  are  homologous.  On  the  contrary,  it  would 
seem  simpler  that  each  part  should  have  been  constructed 
in  such  a  manner  as  to  perform  its  allotted  function  in 
the  best  possible  manner,  without  reference  to  the  rest. 
The  proceedings  of  an  architect,  who  insisted  on  con- 
structing every  building  in  a  town  on  the  plan  of  a 
Gothic  cathedral,  would  not  be  explicable  by  considera- 
tions of  fitness  or  convenience. 

Tn  the  cephalothorax,  the  division  into  somites  is  not 
at  first  obvious,  for,  as  we  have  seen,  the  dorsal  or  tergal 
surface  is  covered  over  by  a  continuous  shield,  distin- 
g  lished  into  thoracic  and  cephalic  regions  only  by  the 
cervical  groove.  Even  here,  however,  when  a  transverse 
section  of  the  thorax  is  compared  with  that  of  the  abdo- 


150        THE  MORPHOLOGY  OF  THE   COMMON   CRAYFISH. 

men  (figs,  15  and  36),  it  will  be  obvious  that  the  tergal 
and  the  sternal  regions  of  the  two  answer  to  one  another ; 
while  the  branchiostegites  correspond  with  greatly  de- 
veloped pleura ;  and  the  inner  wall  of  the  branchial 
chamber,  which  extends  from  the  bases  of  the  appendages 
to  the  attachment  of  the  branchiostegite,  represents  an 
immensely  enlarged  epimeral  region. 

On  examination  of  the  sternal  aspect  of  the  cephalo- 
thorax  the  signs  of  division  into  somites  become  plain 
(figs.  3  and  39,  A).  Between  the  last  two  ambulatory 
limbs  there  is  an  easily  recognisable  sternum  (XIV.), 
though  it  is  considerably  narrower  than  any  of  the 
sterna  of  the  abdominal  somites,  and  differs  from  them 
in  shape. 

The  deep  transverse  fold  which  separates  this  hinder- 
most  thoracic  sternum  from  the  rest  of  the  sternal  wall 
of  the  cephalothorax,  is  continued  upwards  on  the  inner 
or  epimeral  wall  of  the  branchial  cavity ;  and  thus  the 
sternal  and  the  epimeral  portions  of  the  posterior  thoracic 
somite  are  naturally  marked  off  from  those  of  the  more 
anterior  somites. 

The  epimeral  region  of  this  somite  presents  a  very 
curious  structure  (fig.  38).  Immediately  above  the  ar- 
ticular cavities  for  the  appendages  there  is  a  shield- 
shaped  plate,  the  posterior,  convex  edge  of  which  is 
sharp,  prominent,  and  setose.  Close  to  its  upper 
boundary  the  plate  exhibits  a  round  peiforation  (plb.)t 
to  the  margins  of  which  the  stem  of  the  hindermost 


THE   CEPHALOTHORAX.  151 

pleurobranchia  (fig.  4,  plb.  14)  is  attached;  and  in 
front  of  this,  it  is  connected,  by  a  narrow  neck,  with 
an  elongated  triangular  piece,  which  takes  a  vertical 
direction,  and  lies  in  the  fold  which  separates  the  posterior 
thoracic  somite  from  the  next  in  front.  The  base  of  this 


t.xv. 


FIG.  38.  —  Asfaens  fluriaillift. — The  mode  of  cnrupxior  between  the  last 
thoracic  and  the  first  abdominal  somites  (  x  3).  a.  L-shaped  bar  ; 
cpe.  carapace  :  <•./•/>.  14,  coxopodite  of  the  last  ambulatory  leg  ;  plb., 
place  of  attachment,  of  the  pleurobranchia  ;  ft.  A*  V,  sternum,  and 
t.  A~T~  tergum  of  the  first  abdominal  somite. 

piece  unites  with  the  epimeron  of  the  penultimate  somite. 
Its  apex  is  connected  with  the  anterior  end  of  the  horizontal 
arm  of  an  L-shaped  calcified  bar  (fig.  38,  a),  the  upper  end 
of  the  vertical  arm  of  which  is  firmly,  but  moveably,  con- 
nected with  the  anterior  and  lateral  edge  of  the  tergum 
of  the  first  abdominal  somite  (t.  XV.).  The  tendon 'of  one; 


152         THE   MORPHOLOGY   OF   THE   COMMON   CRAYFISH. 

of  the  large  extensor  muscles  of  the  ahdomen  is  attached 
close  to  it. 

The  sternum  and  the  shield-shaped  epimeral  plates 
constitute  a  solid,  continuous!}7  calcified,  ventral  element 
of  the  skeleton,  to  which  the  posterior  pair  of  legs  is 
attached ;  and  as  this  structure  is  united  with  the 
somites  in  front  of  and  behind  it  only  by  soft  cuticle, 
except  where  the  shield-shaped  plate  is  connected,  by 
the  intermediation  of  the  triangular  piece,  with  the 
epimeron  which  lies  in  front  of  it,  it  is  freely  movable 
backwards  and  forwards  on  the  imperfect  hinge  thus 
constituted. 

In  the  same  way,  the  first  somite  of  the  abdomen, 
and,  consequently,  the  abdomen  as  a  whole,  moves  upon 
the  hinges  formed  by  the  union  of  the  L-shaped  pieces 
with  the  triangular  pieces. 

In  the  rest  of  the  thorax,  the  sternal  and  the  epimeral 
regions  of  the  several  somites  are  all  firmly  united 
together.  Nevertheless,  shallow  grooves  answering  to 
folds  of  the  cuticle,  which  run  from  the  intervals 
between  the  articular  cavities  for  the  limbs  towards  the 
tergal  end  of  the  inner  wall  of  the  branchial  chamber, 
mark  off  the  epimeral  portions  of  as  many  somites  as 
there  are  sterna,  from  one  another. 

A  short  distance  above  the  articular  cavities  a  trans- 
verse groove  separates  a  nearly  square  area  of  the  lower 
part  of  the  epimeron  from  the  rest.  Towards  the 
anterior  and  upper  angle  of  this  area,  in  the  two  somites 


THE   CEPHALOTHORAX. 


153 


which  lie  immediately  in  front  of  the  hindermost,  there 
is   a   small   round  aperture   for  the    attachment   of  the 


XIV 


FIG.  39.—  Agtacu*  jlu-viatilix.—  The  cephalothoracic  sterna  and  the  endo- 
phragmal  system  (  x  2).  A,  from  beneath  ;  £,  from  above,  #, «', 
arthrophragms  or  partitions  between  the  articular  cavities  for  the 
limbs  ;  c.ap,  cephalic  apodeme  ;  ct\  cervical  fold  ;  epn.  1,  epimeron  of 
the  antennulary  somite ;  h,  anterior,  and  A',  posterior  horizontal 
process  of  endopleurite :  Ib,  labrum  :  w,  mesophragm ;  mt,  meta- 
stoma  :  p,  paraphragm  ;  1 — XIV,  cephalothoracic  sterna  ;  1 — 14, 
articular  cavities  of  the  cephalothoracic  appendages.  (The  anterior 
cephalic  sterna  are  bent  downwards  in  A  so  as  to  bring  them  into 
the  same  plane  with  the  remaining  cephalothoracic  sterna  ;  in  B 
these  sterna  are  not  shown.) 


154          THE   MORPHOLOGY   OF   THE   COMMON    CRAYFISH. 

rudimentary  branchia.  These  arese  of  the  epimera,  in 
fact,  correspond  with  the  shield-shaped  plate  of  the 
hindermost  somite.  In  the  next  most  anterior  somite 
(that  which  bears  the  first  pair  of  ambulatory  legs)  there 
is  only  a  small  elevation  in  the  place  of  the  rudimentary 
branchia ;  and  in  the  anterior  four  thoracic  somites  no- 
thing of  the  kind  is  visible. 

On  the  sternal  aspect  of  the  thorax  (figs.  3  and  39,  A)  a 
triangular  space  is  interposed  between  the  basal  joints  or 
coxopodites  of  the  penultimate  and  the  ante -penultimate 
pairs  of  ambulatory  legs,  while  the  coxopodites  of  the 
more  anterior  limbs  are  closely  approximated.  The 
triangular  area  in  question  is  occupied  by  two  sterna 
(fig.  39,  A,  XII,  XIII),  the  lateral  margins  of  which  are 
raised  into  flange-like  ridges.  The  next  two  sterna  (X, 
XI)  are  longer,  especially  that  which  lies  between  the 
forceps  (X),  but  they  are  very  narrow ;  while  the  lateral 
processes  are  reduced  to  mere  tubercles  at  the  posterior 
ends  of  the  sterna.  Between  the  three  pairs  of  maxil- 
lipedes,  the  sterna  (VII,  VIII,  IX)  are  yet  narrower,  and 
become  gradually  shorter ;  but  traces  of  the  tubercles  at 
their  posterior  ends  are  still  discernible.  The  most 
anterior  of  these  sternal  rods  passes  into  a  transversely 
elongated  plate,  shaped  like  a  broad  arrow  (V,  VI), 
which  is  constituted  by  the  conjoined  sterna  of  the  two 
posterior  somites  of  the  head. 

Anteriorly  to  this,  and  between  it  and  the  posterior 
end  of  the  elongated  oral  aperture,  the  sternal  region  is 


THE  CEPHALIC   SOMITES.  155 

occupied  only  by  soft  or  imperfectly  calcified  cuticle, 
which,  on  each  side  of  the  hinder  part  of  the  mouth, 
passes  into  one  of  the  lobes  of  the  metastoma  (mt).  At 
the  base  of  each  of  these  lobes  there  is  a  calcified  plate, 
united  by  an  oblique  suture  with  another,  which  occupies 
the  whole  length  of  the  lobe  and  gives  it  firmness.  The 
soft  narrow  lip  which  constitutes  the  lateral  boundary 
of  the  oral  aperture,  and  lies  between  it  and  the  man- 
dible, passes,  in  front,  into  the  posterior  face  of  the 
labrum  (W). 

In  front  of  the  mouth,  the  sternal  region  which  apper- 
tains, in  part,  to  the  antenna?,  and,  in  part,  to  the  man- 
dibles, is  obvious  as  a  broad  plate  (///),  termed  the 
epistoma.  The  middle  third  of  the  posterior  edge  of  the 
epistonia  gives  rise  to  a  thickened  transverse  ridge,  with 
rounded  ends,  slightly  excavated  behind,  and  is  then 
continued  into  the  labrum  (Ib),  which  is  strengthened  by 
three  pairs  of  calcifications,  arranged  in  a  longitudinal 
series.  The  sides  of  the  front  edge  of  the  epistoma  are 
excavated,  and  bound  the  articular  cavities  for  the  basal 
joints  of  the  antennae  (3)  ;  but,  in  the  middle  line,  the 
epistoma  is  continued  forwards  into  a  spear-head  shaped 
process  (figs.  39  and  40,  77),  to  which  the  posterior  end  of 
the  antennulary  sternum  contributes.  The  antennulary 
sternum  is  very  narrow,  and  its  anterior  or  upper  end  runs 
into  a  small  but  distinct  conical  median  spine  (fig.  40,  t.). 
Upon  this  follows  an  uncalcified  plate,  bent  into  the  form  oi 
a  half  cylinder  (/),  which  lies  between  the  inner  ends  of 


156         THE   MORPHOLOGY  OF   THE  COMMON   CRAYFISH. 

the  eye-stalks  and  is  united  with  adjacent  parts  only 
by  flexible  cuticle,  so  that  it  is  freely  movable.  This 
represents  the  whole  of  the  sternal  region,  and  probably 
more,  of  the  ophthalmic  somite. 

The  sterna  of  fourteen  somites  are  thus  identifiable  in 
the    cephalothorax.       The    corresponding    epimera    are 


PCP 


FIG.  ±Q.—Astacm  fluviatilis. — The  ophthalmic  and  antennulary  somites 
(  x  3).  /,  ophthalmic,  and  II,  antennulary  sternum ;  1,  articular 
surface  for  eyestalk  ;  2,  for  antennule  ;  epm,  epimeral  plate  ; 
pup,  procephalic  process  ;  r,  base  of  rostrum  ;  t,  tubercle. 

represented,  in  the  thorax,  by  the  thin  inner  walls  of  the 
branchial  chamber ;  the  pleura,  by  the  branchiostegites  ; 
and  the  terga,  by  so  much  of  the  median  region  of  the 
carapace  as  lies  behind  the  cervical  groove.  That  part  of 
the  carapace  which  is  situated  in  front  of  this  groove  occu- 
pies the  place  of  the  terga  of  the  head ;  while  the  low 
ridge,  skirting  the  oral  and  prae-oral  region,  in  which  it 
terminates  laterally,  represents  the  pleura  of  the  cephalic 
somites. 

The  epimera  of  the  head  are,  for  the  most  part,  very 
narrow ;  but  those  of  the  antennulary  somite  are  broad 
plates  (fig.  40,  epm.),  which  constitute  the  posterior 


THE   ENDOPHRAGMAL    SYSTEM.  157 

wall  of  the  orbits.  I  am  iuclined  to  think  that  a  trans- 
verse ridge,  which  unites  these  under  the  base  of  the 
rostrum,  represents  the  tergum  of  the  antennulary  somite, 
and  that  the.  rostrum  itself  belongs  to  the  next  or 
antennary  somite.* 

The  sharp  convex  ventral  edge  of  the  rostrum  (fig.  41) 
is  produced  into  a  single,  or  sometimes  two  divergent 
spines,  which  descend,  in  front  of  the  ophthalmic  somite, 
towards  the  conical  tubercle  mentioned  above:  it  thus 
gives  rise  to  an  imperfect  partition  between  the  orbits. 


FIG.  41. — Astacm  ftttviatUif.—The  rostrum,  seen  from  the  left  side. 

The  internal  face  of  the  sternal  wall  of  the  whole  of 
the  thorax  and  of  the  post-oral  part  of  the  head,  presents 
a  complicated  arrangement  of  hard  parts,  which  is  known 
as  the  endophraginal  system  (figs.  39,  B,  42,  and  43),  and 
which  performs  the  office  of  an  internal  skeleton  by  afford- 
ing attachment  to  muscles,  and  serving  to  protect  im- 
portant viscera,  while  at  the  same  time  it  ties  the  somites 
together,  and  unites  them  into  a  solid  whole.  In  reality, 
however,  the  curious  pillars  and  bulkheads  which  enter 
into  the  composition  of  the  endophragmal  system  are  all 

*  There  are  some  singular  marine  Crustacea,  the  Sqmllidce,  in  which 
both  the  ophthalmic  and  the  antennary  somites  are  free  and  movable, 
while  the  rostrum  is  articulated  with  the  tergum  of  the  antennary 
somite. 


158       THE  MORPHOLOGY  OF  THE  COMMON   CRAYFISH. 

mere  infoldings  of  the  cuticle,  or  apodemes ;  and,  as  such, 
they  are  shed  along  with  the  other  cuticular  structures 
during  the  process  of  ecdysis. 

Without  entering  into  unnecessary  details,  the  gene- 
ral principle  of  the  construction  of  the  endophragmal 
skeleton  may  he  stated  as  follows.  Four  apodemes  are 
developed  between  every  two  somites,  and  as  every 
apodeme  is  a  fold  of  the  cuticle,  it  follows  that  the 
anterior  wall  of  each  belongs  to  the  somite  in  front,  and 
the  posterior  wall  to  the  somite  behind.  All  four  apodemes 
lie  in  the  ventral  half  of  the  somite  and  form  a  single 
transverse  series  ;  consequently  there  are  two  nearer 
the  middle  line,  which  are  termed  the  endosternites ,  and 
two  further  off,  which  are  the  endopleurites.  The  former 
lie  at  the  inner,  and  the  latter  at  the  outer  ends  of  the 
partitions  or  arthrophragms  (fig.  39,  A,  a,  a',  fig.  42,  apti), 
between  the  articular  cavities  for  the  basal  joints  of  the 
limbs,  and  they  spring  partly  from  the  latter  and  partly 
from  the  sternum  and  the  epiraera  respectively. 

The  endosternite  (fig.  42,  ens.)  ascends  vertically,  with  a 
slight  inclination  forwards,  and  its  summit  narrows  and 
assumes  the  form  of  a  pillar,  with  a  flat,  transversely 
elongated  capital.  The  inner  prolongation  of  the  capital 
is  called  the  mesophragm  (mph.),  the  outer  the  paraphragm 
(pph.).  The  mesophragms  of  the  two  endosternites  of  a 
somite  usually  unite  by  a  median  suture,  and  thus  form 
a  complete  arch  over  the  sternal  canal  (s.c.),  which  lies 
between  the  endosternites. 


THE  EXDOPHRAGMAL   SYSTEM.  159 

The  endopleurites  (en, pi.)  are  also  vertical  plates,  but 
they  are  relatively  shorter,  and  their  inner  angles  give  off 
two  nearly  horizontal  processes,  one  of  which  passes 
obliquely  forwards  (fig.  39,  B,  h,  fig.  42,  h.p.)  and  unites 
with  the  paraphragm  of  the  endosternite  of  the  somite 
in  front,  while  the  other,  passing  obliquely  backwards 
(fig.  39,  /i'),  becomes  similarly  connected  with  the  endo- 
sternite of  the  somite  behind. 


en.pl. 


FIG.  42. — AstacvsfnriatHls.—PL  segment  of  the  endophragmal  system 
( x  3).  aph.  arthrophragm ;  arth,  arthrodial  or  articular  cavity  ; 
cxp,  coxopodite  of  the  ambulatory  leg  ;  enpl,  endopleurite ;  enx, 
endosternite  ;  epm,  epimeron  ;  hp,  horizontal  process  of  endo- 
pleurite ;  -nipTi,  mesophragm  ;  pph,  paraphragm  ;  #.  sternum  of 
somite  ;  gc,  sternal  canal. 

The  endopleurites  of  the  last  thoracic  somite  are  rudi- 
mentary, and  its  endost'ernites  are  small.  On  the  other 
hand,  the  mesophragmal  processes  of  the  endosternites  of 
the  two  posterior  somites  of  the  head  (fig.  39,  B,  c.ap),  by 
which  the  endophragmal  system  terminates  in  front,  are 
particularly  strong  and  closely  united  together.  They 
thus,  with  their  endopleurites,  form  a  solid  partition  be- 
tween the  stomach,  which  lies  upon  them,  and  the  mass  of 


ICO    THE  MORPHOLOGY  OF  THE  COMMON  CRAYFISH. 

coalesced  anterior  thoracic  and  posterior  cephalic  ganglia 
situated  beneath  them.  Strong  processes  are  given  ofl 
from  their  anterior  and  outer  angles,  which  curve  round 
the  tendons  of  the  adductor  muscles  of  the  mandibles,  and 
give  attachment  to  the  abductors. 

In  front  of  the  mouth  there  is  no  such  endophragmal 
system  as  that  which  lies  behind  it.  But  the  anterior  gas- 
tric muscles  are  attached  to  two  flat  calcined  plates,  which 
appear  to  lie  in  the  interior  of  the  head  (though  they  are 
really  situated  in  its  upper  and  front  wall)  on  each  side 
of  the  base  of  the  rostrum,  and  are  called  the  procephnlic 
processes  (tigs.  40,  43,  p.cp).  Each  of  these  plates  con- 
stitutes the  posterior  wall  of  a  narrow  cavity  which  opens 
externally  into  the  roof  of  the  orbit,  and  has  been  regarded 
(though,  as  it  appears  to  me,  without  sufficient  reason)  as 
an  olfactory  organ.  I  am  disposed  to  think,  though  I 
have  not  been  able  to  obtain  complete  evidence  of  the 
fact,  that  the  procephalic  processes  are  the  representa- 
tives of  the  "  procephalic  lobes  "  which  terminate  the 
anterior  end  of  the  body  in  the  embryo  crayfish.  At 
any  rate,  they  occupy  the  same  position  relatively  to  the 
eyes  and  to  the  carapace  ;  and  the  hidden  position  of 
these  processes,  in  the  adult,  appears  to  arise  from  the 
extension  of  the  carapace  at  the  base  of  the  rostrum 
over  the  fore  part  of  the  originally  free  sternal  surface  of 
the  head.  It  has  thus  covered  over  the  procephalic 
processes,  in  which  the  sternal  wall  of  the  body  termi- 
nated; and  the  cavities  which  lie  in  front  of  them  are 


THE  THEORY  OF  THE  SKELETON.        161 

simply  the  interspaces  left  between  the  inferior  or 
posterior  wall  of  the  prolongation  of  the  carapace  and 
the  originally  exposed  external  faces  of  these  regions  of 
the  cephalic  integument. 

Fourteen  somites  having  thus  been  distinguished  in 
the  cephalothorax,  and  six  being  obvious  in  the  abdomen, 
it  is  clear  that  there  is  a  somite  for  every  pair  of  append- 
ages. And,  if  we  suppose  the  carapace  divided  into 
segments  answering  to  these  sterna,  the  whole  body  will 
be  made  up  of  twenty  somites,  each  having  a  pair  of 
appendages.  As  the  carapace,  however,  is  not  actually 
divided  into  terga  in  correspondence  with  the  sterna 
which  it  covers,  all  we  can  safely  conclude  from  the 
anatomical  facts  is  that  it  represents  the  tergal  region  of 
Jhe  somites,  not  that  it  is  formed  by  the  coalescence  of 
primarily  distinct  terga.  In  the  head,  and  in  the  greater 
part  of  the  thorax,  the  somites  are,  as  it  were,  run 
together,  but  the  last  thoracic  somite  is  partly  free  and 
to  a  slight  extent  moveable,  while  the  abdominal  somites 
are  all  free,  and  moveably  articulated  together.  At  the 
anterior  end  of  the  body,  and,  apparently,  from  the  an- 
tennary  somite,  the  tergal  region  gives  rise  to  the 
rostrum,  which  projects  between  and  beyond  the  eyes. 
At  the  opposite  extremity,  the  telson  is  a  corresponding 
median  outgrowth  of  the  last  somite,  which  has  become 
tnoveably  articulated  therewith.  The  narrowing  of  the 
sternal  moieties  of  the  anterior  thoracic  somites,  to- 
12 


162 


THE  MORPHOLOGY   OF   THE   COMMON   CRAYFISH. 


getlier  with  the  sudden  widening  of  the  same  parts  in 
the  posterior  cephalic  somites,  gives  rise  to  the  lateral 
depression  (fig.  39,  cf)  in  which  the  scaphognathite  lies. 
The  limit  thus  indicated  corresponds  with  that  marked 
by  the  cervical  groove  upon  the  surface  of  the  carapace, 
and  separates  the  head  from  the  thorax.  The  three  pair 
of  maxillipedes  (7,  8,  9),  the  forceps  (10),  the  ambulatory 


PlG.  ^.—Astacus  fiumatilis. — Longitudinal  section  of  the  anterior  part 
of  the  cephalothorax  (  x  3).  / — IX,  sterna  of  first  nine  cephalo- 
thoracic  somites;  1,  eyestalk  ;  2,  basal  joint  of  antennule  ;  3,  basal 
joint  of  antenna  ;  4,  mandible  ;  a,  inner  division  of  the  masticatory 
surface  of  the  mandible  ;  «',  apophysisof  the  mandible  for  muscular 
attachment ;  cp,  free  edge  of  carapace  ;  e,  endosternite  :  cnpl.  endo- 
pleurite  ;  epm.  epimeral  plate;  I,  labrum  ;  />/,  muscular  fibres  con- 
necting epimera  with  interior  of  carapace  ;  tut,  metastoma  ;  pep. 
procephalic  process. 


THE  THEORY  OF  THE  SKELETON.        163 

limbs  (11 — 14),  and  the  eight  somites  of  which  they  are 
the  appendages  (VII — XIV),  lie  behind  this  boundary 
and  belong  to  the  thorax.  The  two  pairs  of  maxillae  (5,  6) 
the  mandibles  (4),  the  antenna  (5),  the  antennules  (2), 
the  eyestalks  (1),  and  the  six  somites  to  which  they  are 
attached  (I — VI),  lie  in  front  of  the  boundary  and  com- 
pose the  head. 

Another  important  point  to  be  noticed  is  that,  in  front 
of  the  mouth,  the  sternum  of  the  antennary  somite  (fig. 
43,  ///)  is  inclined  at  an  angle  of  60°  or  70°  to  the  direc- 
tion of  the  sterna  behind  the  mouth.  The  sternum  of  the 
antennulary  somite  (II)  is  at  right  angles  to  the  latter  ;  and 
that  of  the  eyes  (7)  looks  upwards  as  well  as  forwards. 
Hence,  the  front  of  the  head  beneath  the  rostrum,  though 
it  looks  forwards,  or  even  upwards,  is  homologous  with  the 
sternal  aspect  of  the  other  somites.  It  is  for  this  reason 
that  the  feelers  and  the  eyestalks  take  a  direction  so  dif- 
ferent from  that  of  the  other  appendages.  The  change 
of  aspect  of  the  sternal  surface  in  front  of  the  mouth, 
thus  effected,  is  what  is  termed  the  cephalic  flexure. 

Since  the  skeleton  which  invests  the  trunk  of  the  cray- 
fish is  made  up  of  a  twenty-fold  repetition  of  somites, 
homologous  with  those  of  the  abdomen,  we  may  expect 
to  find  that  the  appendages  of  the  thorax  and  of  the  head, 
however  unlike  they  may  seem  to  be  to  those  of  the  ab- 
domen, are  nevertheless  reducible  to  the  same  funda- 
mental plan. 


164- 


THE  MORPHOLOGY   OF   THE   COMMON    CRAYFISH. 


The  third  maxillipede  is  one  of  the  most  complete  of 
these  appendages,  and  may  be  advantageously  made  the 
starting  point  of  the  study  of  the  whole  series. 

PJP 


cxs 


ip- 


cxp 


FIG.  44.—  Astacus  flm-iatilis.—  The  third  or  external  maxillipede  of  the 
left  side  ( x  3).  e,  lamina,  and  br,  branchial  filaments  of  the 
podobranchia  ;  cxp.  coxopodite  ;  cxs,  coxopoditic  setas  ;  bp,  basi- 
podite  ;  ex,  exopodite ;  ip,  ischiopodite  ;  mp,  meropodite ;  cp9 
carpopodite  ;  pp,  propcdite  ;  dp,  dactylopodite. 

Neglecting  details  for  the  moment,  it  may  be  said  that 
the  appendage  consists  of  a  basal  portion  (fig.  44,  cxp,  fy>), 


THE  MAXILLIPEDES.  165 

with  tivo  terminal  divisions  (ip  to  dp,  and  ex),  which  are 
directed  forwards,  below  the  mouth,  and  a  third,  lateral 
appendage  (e,  br),  which  runs  up,  beneath  the  carapace, 
into  the  branchial  chamber.  The  latter  is  the  gill,  or  podo- 
branchia,  attached  to  this  limb,  and  it  is  something  not 
represented  in  the  abdominal  limbs.  But,  with  regard 
to  the  rest  of  the  maxillipede,  it  is  obvious  that  the 
basal  portion  (cxp,  bp)  represents  the  protopodite,  and 
the  two  terminal  divisions  the  endopodite  and  the  exo- 
podite  respectively.  It  has  been  observed  that,  in  the 
abdominal  appendages,  the  extent  to  which  segmentation 
occurs  in  homologous  parts  varies  indefinitely ;  an  endo- 
podite, for  example,  may  be  a  continuous  plate,  or  may 
be  subdivided  into  many  joints.  In  the  maxillipede,  the 
basal  portion  is  divided  into  two  joints ;  and,  as  in  the 
abdominal  limb,  the  first,  or  that  which  articulates  with 
the  thorax,  is  termed  the  coxopodite  (cxp),  while  the  second 
is  the  basipodite  (bp).  The  stout,  leg-like  endopodite 
appears  to  be  the  direct  continuation  of  the  basipodite ; 
while  the  much  more  narrow  and  slender  exopodite  arti- 
culates with  its  outer  side.  The  exopodite  (ex)  is  by  no 
means  unlike  one  of  the  exopodites  of  the  abdominal 
limbs,  consisting  as  it  does  of  an  undivided  base  and  a 
many-jointed  terminal  filament.  The  endopodite,  on  the 
contrary,  is  strong  and  massive,  and  is  divided  into  five 
joints,  named,  from  that  nearest  to  the  base  onwards, 
ischiopodite  (ip),  meropodite  (mp),  carpopodite  (cp),  propo- 
dite  (pp)9  and  dactylopodite  (dp). 


166 


THE  MORPHOLOGY   OF  THE   COMMON   CRAYFISH. 


The  second  maxillipede  (fig.  45,  B)  has  essentially  the 
same  composition  as  the  first,  but  the  exopodite  (ex)  is 
relatively  larger,  the  endopodite  (ip — dp)  smaller  and 
softer;  and,  while  the  ischiopodite  (ip)  is  the  longest 
joint  in  the  third  maxillipede,  it  is  the  meropodite  (mp) 
which  is  longest  in  the  second.  In  the  first  maxillipede 


B 


FIG.  ±5.—Astacmflm-iatilis.—k,  the  first ;  B,  the  second  maxillipede 
of  the  left  side  (  x  3).  c.rp,  coxopodite  ;  bp,  basipodite  ;  e,  br,  po- 
dobranchia  ;  ep,  epipodite  ;  en.  endopodite;  e  y,  exopodite  ;  ^is- 
chiopodite ;  mp.  meropodite  ;  cp,  carpopodite  ;  ppt  propodite  ;  dj>, 
dactylopodite. 

(fig.  45,  A)  a  great  modification  has  taken  place.  The 
coxopodite  (cxp)  and  the  basipodite  (bp)  are  broad  thin 
plates  with  setose  cutting  edges,  while  the  endopodite 
(en)  is  short  and  only  two-jointed,  and  the  undivided 
portion  of  the  exopodite  (ex)  is  very  long.  The  place  of 


PODOBRANCHI^E   AND   EPIPODITES.  167 

the  podobranchia  is  taken  by  a  broad  soft  membranous 
plate  entirely  devoid  of  branchial  filaments  (ep).  Thus, 
in  the  series  of  the  thoracic  limbs,  on  passing  forwards 
from  the.  third  maxillipede,  we  find  that  though  the  plan 
of  the  appendages  remains  the  same  ;  (1)  the  protopodite 
increases  in  relative  size  ;  (2)  the  endopodite  diminishes  ; 
(3)  the  exopodite  increases ;  (4)  the  podobranchia  finally 
takes  the  form  of  a  broad  membranous  plate  and  loses  its 
branchial  filaments. 

Writers  on  descriptive  Zoology  usually  refer  to  the 
parts  of  the  rnaxillipedes  under  different  names  from  those 
which  are  employed  here.  The  protopodite  and  the  endo- 
podite taken  together  are  commonly  called  the  stem  of 
the  maxillipede,  while  the  exopodite  is  the  palp,  and  the 
metamorphosed  podobranchia,  the  real  nature  of  which 
is  not  recognised,  is  termed  the  flagellum. 

When  the  comparison  of  the  maxillipedes  with  the 
abdominal  members,  however,  had  shown  the  funda- 
mental uniformity  of  composition  of  the  two,  it  became 
desirable  to  invent  a  nomenclature  of  the  homologous 
parts  which  should  be  capable  of  a  general  application. 
'The  names  of  protopodite,  endopodite,  exopodite,  which 
I  have  adopted  as  the  equivalents  of  the  "  stem  "  and  the 
"palp,"  were  proposed  by  Milne-Edwards,  who  at  the 
same  time  suggested  epipodite  for  the  "  flagellum."  And 
the  lamellar  process  of  the  first  maxillipede  is  now  very 
generally  termed  an  epipodite  ;  while  the  podobranchias, 
which  liave  exactly  the  same  relations  to  the  following 


168         THE   MOKPHOLOGY    OF   THE   COMMON   CRAYFISH. 

limbs,  are  spoken  of  as  if  they  were  totally  different 
structures,  under  the  name  of  branchiae  or  gills. 

The  flagellum  or  epipodite  of  the  first  maxillipede, 
however,  is  nothing  but  the  slightly  modified  stem  of  a 
podobranchia,  which  has  lost  its  branchial  filaments ; 
but  the  term  "  epipodite  "  may  be  conveniently  used  for 
podobranchise  thus  modified.  Unfortunately,  the  same 
term  is  applied  to  certain  lamelliform  portions  of  the 
branchiae  of  other  Crustacea,  which  answer  to  the  laminae 
of  the  crayfishes'  branchiaa ;  and  this  ambiguity  must  be 
borne  in  mind,  though  it  is  of  no  great  moment. 

On  examining  an  appendage  from  that  part  of  the 
thorax  which  lies  behind  the  third  maxillipede,  say,  for 
example,  the  sixth  thoracic  limb  (the  second  walking  leg) 
(fig.  46),  the  two  joints  of  the  protopodite  and  the  five 
joints  of  the  endopodite  are  at  once  identifiable,  and  so 
is  the  podobranchia;  but  the  exopodite  has  vanished 
altogether.  In  the  eighth,  or  last,  thoracic  limb,  the 
podobranchia  has  also  disappeared.  The  fifth  and 
sixth  limbs  also  differ  from  the  seventh  and  eighth, 
in  being  chelate ;  that  is  to  say,  one  angle  of  the  distal 
end  of  the  propodite  is  prolonged  and  forms  the  fixed  leg 
of  the  pincer.  The  produced  angle  is  that  which  is 
turned  downwards  when  the  limb  is  fully  extended 
(fig.  46).  In  the  forceps,  the  great  chela  is  formed  in 
just  the  same  way ;  the  only  important  difference  lies  in 
the  fact  that,  as  in  the  external  maxillipede,  the  basipo- 
dite  and  the  ischiopodite  are  immoveably  united. 


Y  tG.  46.-  Axtacvx  fotviatilis.—The  second  ambulatory  leg  of  the  left 
side  ^  x  3).  cjrp,  coxopodite  ;  Ip,  basipodite  ;  br,  gill  ;  cxs,  coxo- 
poditic  seta? ;  /°.  lamina  of  gill  or  epipodite  ;  ip,  ischiopodite  ;  wjt?, 
meiopodite  ;  cp,  carpopodite  ;  pp,  propodite  ;  dp,  dactylopodite. 


170         THE  MORPHOLOGY   OF  THE  COMMON   CRAYFISH. 

the  limbs  of  the  thorax  are  all  redupible  to  the  same  type 
as  those  of  the  abdomen,  if  we  suppose  that,  in  the 
posterior  five  pair,  the  exopodites  are  suppressed ;  and 
that,  in  all  but  the  last,  podobranchiae  are  superadded. 

Turning  to  the  appendages  of  the  head,  the  second 
maxilla  (fig.  47,  C)  presents  a  further  modification  of  the 
disposition  of  the  parts  saen  in  the  first  maxillipede. 
The  coxopodite  (cxp)  and  the  basipodite  (bp)  are  still 
thinner  and  more  lamellar,  and  are  subdivided  by  deep 
fissures  which  extend  from  their  inner  edges.  The 
endopodite  (en)  is  very  small  and  undivided.  In  the 
^lace  of  the  exopodite  and  the  epipodite  there  is  only 
one  great  plate,  the  scaphognathite  (sg)  which  either 
is  such  an  epipodite  as  that  of  the  first  maxillipede 
with  its  anterior  basal  process  much  enlarged,  or  repre- 
sents both  the  exopodite  and  the  epipodite.  In  the  first 
maxilla  (B),  the  exopodite  and  the  epipodite  have  dis- 
appeared, and  the  endopodite  (en)  is  insignificant  and 
unjointed.  In  the  mandibles  (A),  the  representative  of 
the  protopodite  is  strong  and  transversely  elongated.  Its 
broad  inner  or  oral  end  presents  a  semicircular  mastica- 
tory surface  divided  by  a  deep  longitudinal  groove  into 
two  toothed  ridges.  The  one  of  these  follows  the  con- 
vex anterior  or  inferior  contour  of  the  masticatory  surface, 
projects  far  beyond  the  other,  and  is  provided  with  a  sharp 
serrated  edge;  the  other  (fig. 43, a)  gives  rise  to  the  straight 
posterior  or  superior  contour  of  the  masticatory  surface, 
and  is  more  obtusely  tuberculated.  In  front,  the  inner 


THE  MANDIBLES   AND   MAXILLA. 


171 


ridge  is  continued  into  a  process  by  which  the  mandible 
articulates  with  the  epistoma  (fig.  47,  A,  ar).  The  endo- 
p 


cxp 


Fm.  47. — AstacvtfluviatUi*. — A,  mandible  ;  B,  first  maxilla  ;  C.  second 
maxilla  of  the  left  side  ( x  3).  ar,  internal,  and  ar',  external 
articular  process  of  the  mandible  ;  bp,  basipodite  ;  cxp,  coxopodite  ; 
en,  endopodite  ;  p,  palp  of  the  mandible ;  sg,  scaphognathite  ;  xt 
internal  process  of  the  first  maxilla, 

podite  is  represented  by  the  three -j ointed  palp  (p),  the 
terminal  joint  of  which  is  oval  and  beset  with  numerous 
strong  setse,  which  are  especially  abundant  along  its 
anterior  edge. 


172        THE  MORPHOLOGY   OF   THE   COMMON   CRAYFISH. 

In  the  antenna  (fig.  48,  C)  the  protopodite  is  two- 
jointed.  The  basal  segment  is  small,  and  its  ventral 
face  presents  the  conical  prominence  011  the  posterior 
aspect  of  which  is  the  aperture  of  the  duct  of  the  renal 
gland  (gg).  The  terminal  segment  is  larger  and  is  subdi- 
vided by  deep  longitudinal  folds,  one  upon  the  dorsal  and 


FIG.  48. — Aftacu*  flwviatilis. — A,  eye-stalk  ;  B,  antennule  ;  C,  antenna 
of  the  left  side  ( x  3).  a,  spine  of  the  basal  joint  of  the  antennule  ; 
#,  corneal  surface  of  the  eye  :  exp,  exopodite  or  squame  of  the 
antenna  ;  gg,  aperture  of  the  duct  of  the  green  gland. 

one  upon  the  ventral  face,  into  two  moieties  which  are 
more  or  less  moveable  upon  one  another.  In  front  and 
externally  it  bears  the  broad  flat  squame  (exp)  of  the  an- 
tenna, as  an  exopodite.  Internally,  the  long  annulated 
"  feeler  "  which  represents  the  endopodite,  is  conne:tel 
with  it  by  two  stout  basal  segments. 


THE  ANTENNULES   AND  THE   EYESTALKS.  173 

The  antennule  (fig.  48,  B)  has  a  three-jointed  stem 
and  two  terminal  annulated  filaments,  the  outer  of  which 
is  thicker  and  longer  than  the  inner,  and  lies  rather  ahove 
as  well  as  external  to  the  latter.  The  peculiar  form  of 
the  hasal  segment  of  the  stem  of  the  antennule  has  already 
been  adverted  to  (p.  116).  It  is  longer  than  the  other 
two  segments  put  together,  and  hear  the  anterior  end 
its  sternal  edge  is  produced  into  a  single  strong  spine  (a). 
The  stem  of  the  antennule  answers  to  the  protopodite  of 
the  other  limhs,  though  its  division  into  three  joints  is 
unusual ;  the  two  terminal  annulated  filaments  represent 
the  endopodite  and  the  exopodite. 

Finally,  the  eyestalk  (A)  has  just  the  same  structure 
as  the  protopodite  of  an  abdominal  limb,  having  a  short 
basal  and  a  long  cylindrical  terminal  joint. 

From  this  brief  statement  of  the  characters  of  the  appen- 
dages, it  is  clear  that,  in  whatever  sense  it  is  allowable  to 
say  that  the  appendages  of  the  abdomen  are  constructed 
upon  one  plan,  which  is  modified  in  execution  by  the 
excess  of  development  of  one  part  over  another,  or  by  the 
suppression  of  parts,  or  by  the  coalescence  of  one  part 
with  another,  it  is  allowable  to  say  that  all  the  appen- 
dages are  constructed  on  the  same  plan,  and  are  modified 
on  similar  principles.  Given  a  general  type  of  appendage 
consisting  of  a  protopodite,  bearing  a  podobranchia,  an 
endopodite  and  .an  exopodite,  all  the  actual  appendages 
are  readily  derivable  from  that  type. 


174         THE  MORPHOLOGY   OF   THE   COMMON  CRAYFISH. 

In  addition,  therefore,  to  their  adaptation  to  the  pur- 
poses which  they  subserve,  the  parts  of  the  skeleton 
of  the  crayfish  show  a  unity  in  diversity,  such  as,  H 
the  animal  were  a  piece  of  human  workmanship,  would 
lead  us  to  suppose  that  the  artificer  was  under  an  obliga- 
tion not  merely  to  make  a  machine  capable  of  doing  cer- 
tain kinds  of  work,  but  to  subordinate  the  nature  and 
arrangement  of  the  mechanism  to  certain  fixed  architec- 
tural conditions. 

The  lesson  thus  taught  by  the  skeletal  organs  is  re- 
iterated and  enforced  by  the  study  of  the  nervous  and  the 
muscular  systems.  As  the  skeleton  of  the  whole  body  is 
capable  of  resolution  into  the  skeletons  of  twenty  separate 
metameres,  variously  modified  and  combined ;  so  is  the 
entire  ganglionic  chain  resolvable  into  twenty  pairs  of 
ganglia  various  in  size,  distant  in  this  region  and 
approximated  in  that ;  and  so  is  the  muscular  system 
of  the  trunk  conceivable  as  the  sum  of  twenty 
myotomes  or  segments  of  the  muscular  system  appro- 
priate to  a  metamere,  variously  modified  according  to 
the  degree  of  mobility  of  the  different  regions  of  the 
organism. 

The  building  up  of  the  body  by  the  repetition  and 
the  modification  of  a  few  similar  parts,  which  is  so  ob- 
vious from  the  study  of  the  general  form  of  the  somites 
and  of  their  appendages,  is  still  more  remarkably  illus- 
trated, if  we  pursue  our  investigations  further,  and  trace 


HISTOLOGY.      TISSUES.  175 

out  the  more  intimate  structure  of  these  parts.  The 
tough,  outer  coat,  which  has  been  termed  the  cuticula, 
except  so  far  as  it  presents  different  degrees  of  hardness, 
from  the  presence  or  absence  of  calcareous  salts,  is 
obviously  everywhere  of  the  same  nature ;  and,  by 
macerating  a  crayfish  in  caustic  alkali,  which  destroys  all 
its  other  components  of  the-  body,  it  will  be  readily 
enough  seen  thai,  a  continuation  of  the  cuticular  layer 
passes  in  at  the  mouth  and  the  vent,  and  lines  the 
alimentary  canal;  furthermore,  that  processes  of  the 
cuticle  covering  various  parts  of  the  trunk  and  limbs 
extend  inwards,  and  afford  surfaces  of  attachment  to  the 
muscles,  as  the  apodemata  and  tendons.  In  technical 
language,  the  cuticular  substance  which  thus  enters  so 
largely  into  the  composition  of  the  bodily  fabric  of  the 
crayfish  is  called  a  tissue. 

The  flesh,  or  muscle,  is  another  kind  of  tissue,  which 
is  readily  enough  distinguished  from  cuticular  tissue  by 
the  naked  eye ;  but,  for  a  complete  discrimination  of 
all  the  different  tissues,  recourse  must  be  had  to  the 
microscope,  the  application  of  which  to  the  study  of 
the  ultimate  optical  characters  of  the  morphological 
constituents  of  the  body  has  given  rise  to  that  branch 
of  morphology  which  is  known  as  Histology. 

If  we  count  every  formed  element  of  the  body,  which 
is  separable  from  the  rest  by  definite  characters,  as  a 
tissue,  there  are  no  more  than  eight  kinds  of  such  tissues 
in  the  crayfish  ;  that  is  to  say,  every  solid  constituent 


176         THE  MORPHOLOGY   OF  THE  COMMON   CRAYFISH. 

of  the  oociy  consists  of  one  or  more  of  the  following  eight 
histological  groups : — 

1.  Blood  corpuscles ;  2.  Epithelium ;  3.  Connective 
tissue;  4.  Muscle ;  5.  Nerve;  6.  Ova;  7.  Spermatozoa; 
8.  Cuticle. 

1.  A  drop  of  freshly-drawn  blood  of  the  crayfish  con- 
tains multitudes  of  small  particles,  the  blood  corpuscles, 


FIG.  49. — Astacus  JlwriatUif. — The  corpuscles  of  the  blood,  highly 
magnified.  1 — 8,  show  the  changes  undergone  by  a  single  cor- 
puscle during  a  quarter  of  an  hour  ;  n,  the  nucleus  ;  9  and  10 
are  corpuscles  killed  by  magenta,  and  'having  the  nucleus  deeply 
stained  by  the  colouring  matter. 


which  rarely  exceed  l-700th,  and  usually  are  about 
l-1000th,  of  an  inch  in  diameter  (fig.  49).  They 
are  sometimes  pale  and  delicate,  but  generally  more  or 
less  dark,  from  containing  a  number  of  minute  strongly 
refracting  granules,  and  they  are  ordinarily  exceedingly 
irregular  in  form.  If  one  of  them  is  watched  continu- 


EPITHELIUM.  177 

ously  for  two  or  three  minutes,  its  shape  will  be  seen  to 
undergo  the  constant  but  slow  changes  to  which  passing 
reference  has  alreacly  been  made  (p.  69).  One  or  other  of 
the  irregular  prolongations  will  be  drawn  in,  and  another 
thrown  out  elsewhere.  The  corpuscle,  in  fact,  has  an 
inherent  contractility,  like  one  of  those  low  organisms, 
known  as  an  Amoeba,  whence  its  motions  are  frequently 
called  am<xbiform.  In  its  interior,  an  ill-marked  oval 
contour  may  be  seen^  indicating  the  presence  of  a  sphe- 
roidal body,  about  1 -2000th  of  an  inch  in  diameter,  which 
is  the  nucleus  of  the  corpuscle  (n).  The  addition  of  some 
re-agents,  such  as  dilute  acetic  acid,  causes  the  corpuscles 
at  once  to  assume  a  spherical  shape,  and  renders  the  nuc- 
leus very  conspicuous  (fig.  49,  9  and  10).  The  blood 
corpuscle  is,  in  fact,  a  simple  nucleated  cell,  composed 
of  a  contractile  protoplasmic  mass,  investing  a  nucleus  ; 
it  is  suspended  freely  in  the  blood;  and,  though  as 
much  a  part  of  the  crayfish  organism  as  any  other  of 
its  histological  elements,  leads  a  quasi-independent  ex- 
istence in  that  fluid. 

2.  Under  the  general  name  of  epithelium,  may  be  in- 
cluded a  form  of  tissue,  which  everywhere  underlies  the 
exoskeleton  (where  it  corresponds  with  the  epidermis  of  the 
higher  animals),  and  the  cuticular  lining  of  the  alimen- 
tary canal,  extending  thence  into  the  hepatic  caeca.  It  is 
further  met  with  in  the  generative  organs,  and  in  the  green 
gland.  Where  it  forms  the  subcuticular  layer  of  the 

integument  and  of  the  alimentary  canal,  it  is  found  to 
13 


178     THE   MORPHOLOGY   OF   THE  COMMON   CRAYFISH. 

consist  of  a  protoplasmic  substance  (fig.  50),  in  which  close 
set  nuclei  (n)  are  imbedded.  If  a  number  of  blood  corpus- 
cles could  be  supposed  to  be  closelty  aggregated  together 
into  a  continuous  sheet,  they  would  give  rise  to  such  a 
structure  as  this ;  and  there  can  be  no  doubt  that  it 
really  is  an  aggregate  of  nucleated  cells,  though  the 
limits  between  the  individual  cells  are  rarely  visible  in  the 
fresh  state.  In  the  liver,  however,  the  cells  grow,  and 
become  detached  from  one  another  in  the  wider  and  lower 


FlG.  50. — Astaciis  Jhtviatili*. — Epithelium,  from  the  epidermic  laye.2 
subjacent  to  the  cuticle,  highly  magnified.  A,  in  vertical  section  ; 
£,  from  the  surface,  n,  nuclei. 

parts  of  the  ca3ca,  and  their  essential  nature  is  thus 
obvious. 

3.  Immediately  beneath  the  epithelial  layer  follows  a 
tissue,  disposed  in  bands  or  sheets,  which  extend  to  the 
subjacent  parts,  invest  them,  and  connect  one  with 
another.  Hence  this  is  called  connective,  tissue. 

The  connective  tissue  presents  itself  under  three  forms. 
In  the  first  there  is  a  transparent  homogeneous-looking 
matrix,  or  ground  substance,  through  which  are  scattered 
many  nuclei.  In  fact,  this  form  of  connective  tissue 


CONNECTIVE  TISSUE. 


179 


very  closely  resembles  the  epithelial  tissue,  except  that 
the  intervals  between  the  nuclei  are  wider,  and  that  the 
substance  in  which  they  are  imbedded  cannot  be  broken 
up  into  a  separate  cell-body  for  each  nucleus.  In  the 
second  form  (fig.  51,  A)  the  matrix  exhibits  fine  wavy 
parallel  lines,  as  if  it  were  marked  out  ii .t  >  imperfect 


FIG.  ol. — Astacus  fluriutiUx. — Connective  tissue  ;  A,  second  form  ;  £, 
third  form.     a.  cavities  ;  n,  nuclei.     Highly  magnified. 

fibres.  In  this  form,  as  in  the  next  to  be  described, 
more  or  less  spherical  cavities,  which  contain  a  clear 
fluid,  are  excavated  in  the  matrix ;  and  the  number  of 


180     THE  MORPHOLOGY  OF  THE   COMMON   CRAYFISH. 

these  is  sometimes  so  great,  that  the  matrix  is  propor- 
tionally very  much  reduced,  and  the  structure  acquires  a 
close  superficial  similarity  to  that  of  the  parenchyma  of 
plants.  This  is  still  more  the  case  with  a  third  form,  in 
which  the  matrix  itself  is  marked  off  into  elongated  or 
rounded  masses,  each  of  which  has  a  nucleus  in  its 
interior  (fig.  51,  jB).  Under  one  form  or  another,  the 
connective  tissue  extends  throughout  the  body,  enshenth- 
ing  the  various  organs,  and  forming  the  walls  of  the  blood 
sinuses. 

The  third  form  is  particularly  abundant  in  the  outer 
investment  of  the  heart,  the  arteries,  the  alimentary 
canal,  and  the  nervous  centres.  About  the  cerebral  and 
anterior  thoracic  ganglia,  and  on  the  exterior  of  the 
heart,  it  usually  contains  more  or  less  fatty  matter.  In 
these  regions,  many  of  the  nuclei,  in  fact,  are  hidden  by 
the  accumulation  round  them  of  granules  of  various 
sizes,  some  of  which  are  composed  of  fat,  while  others 
consist  of  a  proteinaceous  material.  These  aggregates 
of  granules  are  usually  spheroidal ;  and,  with  the  matrix  in 
which  they  are  imbedded  and  the  nucleus  which  they  sur- 
round, they  are  often  readily  detached  when  a  portion  of 
the  connective  tissue  is  teased  out,  and  are  then  known  as 
fat  cells.  From  what  has  been  said  respecting  the  dis- 
tribution of  the  connective  tissue,  it  is  obvious  that  if 
all  the  other  tissues  could  be  removed,  this  tissue  would 
form  a  continuous  whole,  and  represent  a  sort  of  model, 
or  cast,  of  the  whole  body  of  the  crayfish. 


MUSCULAR    TISSUE. 


181 


4.  The  muscular  tissue  of  the  crayfish  always  has  the 
form  of  bands  or  fibres,  of  very  various  thickness,  marked, 
when  viewed  by  transmitted  light,  by  alternate  darker  and 


FIG.  52. — Astacns  flHtiatilis. — A,  a  single  muscular  fibre,  transverse 
diameter  TLth  of  an  inch  ;  B,  a  portion  of  the  same  more  highly 
magnified  ;  C,  a  smaller  portion  treated  with  alcohol  and  acetic 
acid  still  more  highly  magnified  ;  D  and  E,  the  splitting  up  of  a 
part  of  a  fibre,  treated  with  picro-carmine,  into  fibrillae  ;  F,  the 
connection  of  a  nervous  with  a  muscular  fibre  which  has  been 
treated  with  alcohol  and  acetic  acid,  a,  darker,  and  #,  clearer  portions 
of  the  fibrillas  ;  «,  nuclei  :  ni\  nerve  fibre  ;  <<?,  sarcolemma  ;  t,  tendon  ; 
1 — 5.  successive  dark  granular  striae  answering  to  the  granular 
portions,  a,  of  each  fibrilla. 


182     THE  MORPHOLOGY   OF   THE   COMMON   CRAYFISH. 

lighter  striae,  transversely  to  the  axis  of  the  fibres 
(fig.  52  A).  The  distance  pf  the  transverse  striae  from  one 
nnother  varies  with  the  condition  of  the  muscle,  from 
1-4, 000th  of  an  inch  in  the  quiescent  state  to  as  little  as 
1-30, 000th  of  an  inch  in  that  of  extreme  contraction. 
The  more  delicate  muscular  fibres,  like  those  of  the 
heart  and  those  of  the  intestine,  are  imbedded  in  the 
connective  tissue  of  the  organ,  but  have  no  special  sheaths. 


IS 
S3, 


FIG.  53. — Astacns  fomatilh. — A,  living  muscular  fibres  very  highly 
magnified  ;  £,  a  fibrilla  treated  with  solution  of  sodium  chloride  ; 
C,  a  fibrilla  treated  with  strong  nitric  acid,  s,  septal  lines  ;  .v, 
septal  zones  ;  w,  interseptal  zones  ;  a,  transverse  line  in  the  inter- 
septal  zone. 

The  fibres  which  make  up  the  more  conspicuous  muscles 
of  the  trunk  and  limbs,  on  the  other  hand,  are  much 
larger,  and  are  invested  by  a  thin,  transparent,  structure- 
less sheath,  which  is  termed  the  sarcolemma.  Nuclei 
are  scattered,  at  intervals,  through  the  striated  substance 
of  the  muscle  ;  and,  in  the  larger  muscular  fibres,  a  layer 
of  nucleated  protoplasm  lies  between  the  sarcolemma  and 
the  striated  muscle  substance. 


MUSCULAR   TISSUE.  183 

This  much  is  readily  seen  in  a  specimen  of  muscular 
fibre  taken  from  any  part  of  the  body,  and  whether  alive 
or  dead.  But  the  results  of  the  ultimate  optical  analysis 
of  these  appearances,  and  the  conclusions  respecting  the 
normal  structure  of  striped  muscle  which  may  be  legiti- 
mately drawn  from  them,  have  been  the  subjects  of  much 
controversy. 

Quiescent  muscular  fibres  from  the  chela  of  the  forceps 
of  a  crayfish,  examined  while  still  living,  without  the 
addition  of  any  extraneous  fluid,  and  with  magnifying 
powers  of  not  less  than  seven  or  eight  hundred  diameters, 
exhibit  the  following  appearance.  At  intervals  of  about 
1 -4000th  of  an  inch,  very  delicate  but  dar%  and  well- 
defined  transverse  lines  are  visible ;  and  these,  on  careful 
focussing,  appear  beaded,  as  if  they  were  made  of  a  series 
of  close-set  minute  granules  not  more  than  1 -20,000th 
to  1 -30,000th  of  an  inch  in  diameter.  These  may  be 
termed  the  septal  lines  (fig.  52,  D  and  E,  a;  C,  1 — 5 ; 
fig.  53,  s).  On  each  side  of  every  septal  line  there 
is  a  very  narrow  perfectly  transparent  band,  which  may 
be  distinguished  as  the  septal  zone  (fig.  53,  sz).  Upon 
this  follows  a  relativety  broad  band  of  a  substance  which 
has  a  semi-transparent  aspect,  like  very  finely  ground 
glass,  and  hence  appeal's  somewhat  dark  relatively  to  the 
septal  zone.  Upon  this  inter-septal  zone  (i  s)  follows 
another  septal  zone,  then  a  septal  line,  another  septal 
zone,  an  inter-septal  zone,  and  so  on  throughout  the 
whole  length  of  the  fibre. 


184    THE   MOEPHOLOGY   OF   THE   COMMON   CRAYFISH. 

In  the  perfectly  unaltered  state  of  the  muscle  no  other 
transverse  markings  than  these  are  discernible.  But  it  is 
always  possible  to  observe  certain  longitudinal  markings  ; 
and  these  are  of  three  kinds.  In  the  first  place,  the  nuclei 
which,  in  the  perfectly  fresh  muscle,  are  delicate  trans- 
parent oval  bodies,  are  lodged  in  spaces  which  taper  off  at 
each  end  into  narrow  longitudinal  clefts  (fig.  52,  A,  B).  Pro- 
longations of  the  protoplasmic  sheath  of  the  fibre  extend 
inwards  and  fill  these  clefts.  Secondly,  there  are  similar 
clefts  interposed  between  these,  but  narrow  and  merely 
linear  throughout.  Sometimes  these  clefts  contain  fine 
granules.  Thirdly,  even  in  the  perfectly  fresh  muscle, 
extremely  faint  parallel  longitudinal  striae  1-7, 000th 
of  an  inch,  or  thereabouts,  apart,  traverse  the  several 
zones,  so  that  longer  or  shorter  segments  of  the 
successive  septal  lines  are  inclosed  between  them.  A 
transverse  section  of  the  muscle  appears  divided  into 
rounded  or  polygonal  areas  of  the  same  diameter,  sepa- 
rated from  one  another  here  and  there  by  minute  inter- 
stices. Moreover,  on  examination  of  perfectly  fresh 
muscle  with  high  magnifying  powers,  the  septal  lines  are 
hardly  ever  straight  for  any  distance,  but  are  broken  up 
into  short  lengths,  which  answer  to  one  or  more  of  the 
longitudinal  divisions,  and  stand  at  slightly  different 
heights. 

The  only  conclusion  to  be  drawn  from  these  appear- 
ances seems  to  me  to  be  that  the  substance  of  the  muscle 
is  composed  of  distinct  fibrils  ;  and  that  the  longitudinal 


MUSCULAR  TISSUE.  185 

and  the  rounded  areae  of  the  transverse  section  are 
simply  the  optical  expressions  of  the  boundaries  of  these 
fibrils.  In  the  perfectly  unaltered  state  of  the  tissue, 
however,  the  fibrils  are  so  closely  packed  that  their 
boundaries  are  scarcely  discernible. 

Thus  each  muscular  fibre  may  be  regarded  as  com- 
posed of  larger  and  smaller  bundles  of  fibrils  im- 
bedded in  a  nucleated  protoplasmic  framework  which 
ensheaths  the  whole  and  is  itself  invested  by  the  sar- 
coleinma. 

As  the  fibre  dies,  the  nuclei  acquire  hard,  dark  contours 
and  their  contents  become  granular,  while  at  the  same 
time  the  fibrils  acquire  sharp  and  well-defined  boundaries. 
In  fact,  the  fibre  may  now  be  readily  teased  out  with 
needles,  and  the  fibrils  isolated. 

In  muscle  which  has  been  treated  with  various  reagents, 
such  as  alcohol,  nitric  acid,  or  solution  of  common  salt, 
the  fibrils  themselves  may  be  split  up  into  filaments  of 
extreme  tenuity,  each  of  which  appears  to  answer  to 
one  of  the  granules  of  the  septal  lines.  Such  an 
isolated  muscle  filament  looks  like  a  very  fine  thread 
carrying  minute  beads  at  regular  intervals. 

The  septal  lines  resist  most  reagents,  and  remain 
visible  in  muscular  fibres  which  have  been  subjected  to 
various  modes  of  treatment ;  but  they  may  have  the 
appearance  of  continuous  bars,  or  be  more  or  less  com- 
pletely resolved  into  separate  granules,  according  to  cir- 
cumstances. On  the  other  hand,  what  is  to  be  seen  in 


186    THE   MORPHOLOGY  OF  THE  COMMON   CRAYFISH. 

the  interspace  between  every  two  septal  lines  depends 
upon  the  reagent  employed.  With  dilute  acids  and 
slrong  solutions  of  salt,  the  inter-septal  substance  swells 
up  and  becomes  transparent,  so  that  it  ceases  to  be  dis- 
tinguishable from  the  septal  zone.  At  the  same  time  a 
distinct  but  faint  transverse  line  may  appear  in  the 
middle  of  its  length.  Strong  nitric  acid,  on  the  con- 
trary, renders  the  inter-septal  substance  more  opaque, 
and  the  septal  zones  consequently  appear  very  well 
denned. 

In  living  and  recently  dead  muscle,  as  well  as  in 
muscles  which  have  been  preserved  in  spirit  or  hardened 
with  nitric  acid,  the  inter-septal  zones  polarize  light;  and 
hence,  in  the  dark  field  of  the  polarizing  microscope,  the 
fibre  appears  crossed  by  bright  bands,  which  correspond 
with  the  inter-septal  zones,  or  at  any  rate,  with  the 
middle  parts  of  them.  The  substance  wThich  forms  the 
septal  zones,  on  the  contrary,  produces  no  such  effect, 
and  consequently  remains  dark;  while  the  septal  lines 
again  have  the  same  property  as  the  inter-septal  sub- 
stance, though  in  a  less  degree. 

In  fibres  which  have  been  acted  upon  by  solution  o 
salt,  or  dilute  acids,  the  inter-septal  zones  have  lost 
their  polarizing  property.  As  we  know  that  the  reagents 
in  question  dissolve  the  peculiar  constituent  of  muscle, 
myosin,  it  is  to  be  concluded  that  the  inter-septal  sub- 
stance is  chiefly  composed  of  myosin. 

Thus  a  fibril  may  be  considered  to  be   made   up  of 


NERVOUS  TISSUE.  187 

segments  of  different  material  arranged  in  regular  order ; 
S — sz — IS — sz — S — sz — IS — sz — S  :  S  representing  the 
septal  line  ;  sz,  the  septal  zone  ;  IS,  the  inter-septal  zone. 
Of  these,  IS  is  the  chief  if  not  the  only  seat  of  the 
myosin ;  what  the  composition  of  sz  and  of  S  may  be 
is  uncertain,  but  the  supposition,  that,  in  the  living 
muscle,  sz  is  a  mere  fluid,  appears  to  me  to  be  wholly 
inadmissible.  , 

When  living  muscle  contracts,  the  inter-septal  zones 
become  shorter  and  wider  and  their  margins  darker, 
while  the  septal  zones  and  the  septal  lines  tend  to 
become  effaced — as  it  appears  to  me  simply  in  conse- 
quence of  the  approximation  of  the  lateral  margins  of 
the  inter-septal  zones.  It  is  probable  that  the  sub- 
stance of  the  intermediate  zone  is  the  chief,  if  not  the 
only,  seat  of  the  activity  of  the  muscle  during  con- 
traction. 

5.  The  elements  of  the  nervous  tissue  are  of  two  kinds, 
nerve-cells,  and  nerve  fibres  ;  the  former  are  found  in  the 
ganglia,  and  they  vary  very  much  in  size  (fig.  54,  B).  Each 
ganglionic  corpuscle  consists  of  a  cell  body  produced 
into  one  or  more  processes  which  sometimes,  if  not 
always,  end  in  nerve  fibres.  A  large,  clear  spherical 
nucleus  is  seen  in  the  interior  of  the  nerve-cell ;  and 
in  the  centre  of  this  is  a  well  defined,  small  round 
particle,  the  nuckolus.  The  corpuscle,  when  isolated, 
is  often  surrounded  by  a  sort  of  sheath  of  small  nucle- 
ated cells. 


188    THE   MORPHOLOGY   OF   THE   COMMON    CRAYFISH. 

The  nerve  fibres  (fig.  55)  of  the  crayfish  are  remarkable 
for  the  large  size  which  some  of  them  attain.  In  the 
central  nervous  system  a  few  reach  as  much  as  1- 200th  of 
an  inch  in  diameter;  and  fibres  of  l-300th  or  l-400th  of 


FIG.  54. — Agtawis flttviatlli*. — A,  one  of  the  (double)  abdominal  gan- 
glia, with  the  nerves  connected  with  it  (  x  25)  ;  B,  a  nerve  cell  or 
ganglionic  corpuscle  (  x  250).  #,  sheath  of  the  nerves  ;  c,  sheath 
of  the  ganglion  ;  co,  co'.  commissural  cords  connecting  the  ganglia 
with  those  in  front,  and  those  behind  them.  gl.c.  points  to  the 
ganglionic  corpuscles  of  the  ganglia  ;  n,  nerve  fibres. 

an  inch  in  diameter  are  not  rare  in  the  main  branches. 
Each  fibre  is  a  tube,  formed  of  a  strong  and  elastic,  some- 
times fibrillated,  sheath,  in  which  nuclei  are  imbedded 
at  irregular  intervals  ;  and,  when  the  nerve  trunk  gives 


NERVOUS   TISSUE.  189 

off  a  branch,  more  or  fewer  of  tnese  tubes  divide,  sending 
off  a  prolongation  into  each  branch. 

When  quite  fresh,  the  contents  of  the  tubes  are  per- 
fectly pellucid,  and  without  the  least  indication  of  struc- 
ture ;  and,  from  the  manner  in  which  the  contents 


FlG.  55. — Astacus  fluvia tills. — Three  nerve  fibres,  with  the  connective 
tissue  in  which  they  are  imbedded  (magnified  about  250  diameters) ; 
n.  nuclei. 

exude  from  the  cut  ends  of  the  tubes,  it  is  evident  that 
they  consist  of  a  fluid  of  gelatinous  consistency.  As  the 
fibre  dies,  and  under  the  influence  of  water  and  of  many 
chemical  re-agents,  the  contents  break  up  into  globules 
or  become  turbid  and  finely  granular. 

Where  motor  nerve  fibres  terminate  in  the  muscles  to 
which  they  are  distributed,  the  sheath  of  each  fibre 
becomes  continuous  with  the  sarcolernma  of  the  muscle, 
and  the  subjacent  protoplasm  is  commonly  raised  into  a 
small  prominence  which  contains  several  nuclei  (fig.  52,  F). 
These  are  called  the  terminal  or  motor  plates. 


190     THE   MORPHOLOGY   OF  THE   COMMON   CRAYFISH. 

6,  7.  The  ova  and  the  spermatozoa  have  already  been 
described  (pp.  132—135). 

It  will  be  observed  that  the  blood  corpuscles,  the 
epithelial  tissues,  the  ganglionic  corpuscles,  the  ova 
and  the  spermatozoa,  are  all  demonstrably  nucleated 
cells,  more  or  less  modified.  The  first  form  of  con- 
nective tissue  is  so  similar  to  epithelial  tissue,  that  it  may 
obviously  be  regarded  as  an  aggregate  of  as  many  cells  as 
it  presents  nuclei,  the  matrix  representing  the  more  or 
less  modified  and  confluent  bodies  of  the  cells,  or  products 
of  these.  But  if  this  be  so,  then  the  second  and  third 
forms  have  a  similar  composition,  except  so  far  as  the 
matrix  of  the  cells  has  become  fibrillated,  or  vacuolated, 
or  marked  off  into  masses  corresponding  with  the  several 
nuclei.  By  a  parity  of  reasoning,  muscular  tissue  may 
also  be  considered  a  cell  aggregate,  in  which  the  inter- 
nuclear  substance  has  become  converted  into  striated 
muscle ;  while,  in  the  nerve  fibres,  a  like  process  of 
metamorphosis  may  have  given  rise  to  the  pellucid 
gelatinous  nerve  substance.  But,  if  we  accept  the 
conclusions  thus  suggested  by  the  comparison  of  the 
various  tissues  with  one  another,  it  follows  that  every 
histological  element,  which  has  now  .been  mentioned, 
is  either  a  simple  nucleated  cell,  a  modified  nucleated 
cell,  or  a  more  or  less  modified  cell  aggregate.  In 
other  words,  every  tissue  is  resolvable  into  nucleated 
cells. 


FIG.  56. — Astacm  fluviatilis. — The  structure  of  the  cuticle.  A.  trans- 
verse section  of  a  joint  of  the  forceps  (  x  4)  ;  s,  setae  ;  B,  a  por- 
tion of  the  same  (  x  30)  ;  C,  a  portion  of  B  more  highly  magnified, 
«,  epiostracum  ;  b,  ectostracum  ;  c,  endostracum  ;  d,  canal  of  seta  ; 
e,  canals  filled  with  air  ;  s,  seta.  D,  section  of  an  intersternal 
membrane  of  the  abdomen,  the  portion  to  the  right  in  the  natural 
condition,  the  remainder  pulled  apart  with  needles  (  x  20)  ;  E, 
small  portion  of  the  same,  highly  magnified  ;  a,  intermediate  sub- 
Stance  ;  &,  laminae.  F,  a  seta,  highly  magnified  ;  a  and  b,  joints. 


192     THE  MORPHOLOGY  OF  THE   COMMON   CRAYFISH. 

A  notable  exception  to  this  generalisation,  howev  jr, 
obtains  in  the  case  of  the  cuticular  structures,  in  which 
no  cellular  components  are  discoverable.  In  its  simplest 
form,  such  as  that  presented  by  the  lining  of  the  in- 
testine, the  cuticle  is  a  delicate,  transparent  membrane, 
thrown  off  from  the  surface  of  the  subjacent  cells,  either 
by  a  process  of  exudation,  or  by  the  chemical  transfor- 
mation of  their  superficial  layer.  No  pores  are  discern- 
ible in  this  membrane,  but  scattered  over  its  surface 
there  are  oval  patches  of  extremely  minute,  sharp  conical 
processes,  which  are  rarely  more  than  1-5, 000th  of  an 
inch  long.  Where  the  cuticle  is  thicker,  as  in  the 
stomach  and  in  the  exoskeleton,  it  presents  a  stratified 
appearance,  as  if  it  were  composed  of  a  number  of  laminae, 
of  varying  thickness,  which  had  been  successively  thrown 
off  from  the  subjacent  cells. 

Where  the  cuticular  layer  of  the  integument  is  un- 
calcified,  for  example,  between  the  sterna  of  the  abdo- 
minal somites,  it  presents  an  external,  thin,  dense, 
wrinkled  lamina,  the  epiostracum,  followed  by  a  soft 
substance,  which,  on  vertical  section,  presents  numerous 
alternately  more  transparent  and  more  opaque  bands, 
which  run  parallel  with  one  another  and  with  the  free 
surfaces  of  the  slice  (fig.  56,  D).  These  bands  are  very 
close-set,  often  not  more  than  l-5000th  of  an  inch  apart 
near  the  outer  and  the  inner  surfaces,  but  in  the  middle 
)f  the  section  they  are  more  distant. 

If  a  thin  vertical  slice  of  the  soft  cuticle   is  gently 


CUTICULAR  TISSUE.  193 

pulled  with  needles  in  the  direction  of  its  depth,  it 
stretches  to  eight  or  ten  times  its  previous  diameter, 
the  clear  intervals  between  the  dark  bands  becoming 
proportionally  enlarged,  especially  in  the  middle  of  the 
slice,  while  the  dark  bands  themselves  become  apparently 
thinner,  and  more  sharply  denned.  The  dark  bands 
may  then  be  readily  drawn  to  a  distance  of  as  much  as 
l-300th  of  an  inch  from  one  another ;  but  if  the  slice  is 
stretched  further,  it  splits  along,  or  close  to,  one  of  the 
dark  lines.  The  whole  of  the  cuticular  layer  is  stained 
by  such  colouring  matters  as  hsematoxylin ;  and,  as  the 
dark  bands  become  more  deeply  coloured  than  the  inter- 
mediate transparent  substance,  the  transverse  stratifi- 
cation is  made  very  manifest  by  this  treatment. 

Examined  with  a  high  magnifying  power,  the  trans- 
parent substance  is  seen  to  be  traversed  by  close-set, 
faint,  vertical  lines,  while  the  dark  bands  are  shown  to 
be  produced  by  the  cut  edges  of  delicate  laminae,  having 
a  finely  striated  appearance,  as  if  they  were  composed 
of  delicate  parallel  wavy  fibrillse. 

In  the  calcined  parts  of  the  exoskeleton  a  thin,  tough, 
wrinkled  epiostracum  (fig.  56,  B,  a),  and,  subjacent  to 
this,  a  number  of  alternately  lighter  and  darker  strata 
are  similarly  discernible :  though  all  but  the  innermost 
laminae  are  hardened  by  a  deposit  of  calcareous  salts, 
which  are  generally  evenly  diffused,  but  sometimes  take 
the  shape  of  rounded  masses  with  irregular  contours. 

Immediately  beneath  the  epiostracum  there  is  a  zone 
14 


l!)i       THE   MORPHOLOGY   OF   THE   COMMON    CRAYFISH. 

which  may  occupy  a  sixth  or  a  seventh  of  the  thickness 
of  the  whole,  which  is  more  transparent  than  the  rest, 
and  often  presents  hardly  any  trace  of  horizontal  or 
vertical  striation.  When  it  appears  laminated,  the  strata 
are  very  thin.  This  zone  may  be  distinguished  as  the 
ectostraciun  (b),  from  the  endostracum  (c),  which  makes 
up  the  rest  of  the  exoskeleton.  In  the  outer  part  of  the 
endostracum,  the  strata  are  distinct,  and  may  be  as  much 
as  1 -500th  of  an  inch  thick,  but  in  the  inner  part  they 
become  very  thin,  and  the  lines  which  separate  them 
may  be  not  more  than  1- 8000th  of  an  inch  apart. 
Fine,  parallel,  close-set,  vertical  striaB  (e)  traverse  all  the 
strata  of  the  endostracum,  and  may  usually  be  traced 
through  the  ectostracum,  though  they  are  always  faint, 
and  sometimes  hardly  discernible,  in  this  region.  When 
a  high  magnifying  power  is  employed,  it  is  seen  that 
these  striae,  which  are  about  1- 7000th  of  an  inch  apart, 
are  not  straight,  but  that  they  present  regular  short  un- 
dulations, the  alternate  convexities  and  concavities  of 
which  correspond  with  the  light  and  the  dark  bands 
respectively. 

If  the  hard  exoskeleton  has  been  allowed  to  become 
partially  or  wholly  dry  before  the  section  is  made,  the 
latter  will  look  white  by  reflected  and  black  by  trans- 
mitted light,  in  consequence  of  the  places  of  the  striae 
being  taken  by  threads  of  air  of  such  extreme  tenuity, 
that  they  may  measure  not  more  than  1-30, 000th  of  an 
inch  in  diameter.  It  is  to  be  concluded,  therefore,  thai 


CUTICULAR  TISSUE.  195 

the  striae  are  the  optical  indications  of  parallel  undulating 
canals  which  traverse  the  successive  strata  of  the  cuticle, 
and  are  ordinarily  occupied  by  a  fluid.  When  this  dries 
up,  the  surrounding  air  enters,  and  more  or  less  com- 
pletely fills  the  tubes.  And  that  this  is  really  the  case 
may  be  proved  \)y  making  very  thin  sections  parallel  with 
the  face  of  the  exoskeleton,  for  these  exhibit  innumerable 
minute  perforations,  set  at  regular  distances  from  one 
another,  which  correspond  with  the  intervals  between  the 
striae  in  the  vertical  section  ;  and  sometimes  the  contours 
of  the  areaa  which  separate  the  apertures  are  so  well 
defined  as  to  suggest  a  pavement  of  minute  angular 
blocks,  the  corners  of  which  do  not  quite  meet. 

\Yhen  a  portion  of  the  hard  exoskeleton  is  decalcified, 
a  chitinous  substance  remains,  which  presents  the  same 
structure  as  that  just  described,  except  that  the  epios- 
tracum  is  more  distinct ;  while  the  ectostracurn  appears 
made  up  of  very  thin  laminaa,  and  the  tubes  are  repre- 
sented by  delicate  striae,  which  appear  coarser  in  the 
iv^ion  of  the  dark  zones.  As  in  the  naturally  soft  pails 
of  the  exoskeleton,  the  decalcified  cuticle  may  be  split 
into  flakes,  and  the  pores  are  then  seen  to  be  disposed 
in  distinct  areae  circumscribed  by  clear  polygonal  borders. 
These  perforated  arese  appear  to  correspond  with  indi- 
vidual cells  of  the  ectoderm,  and  the  canals  thus  answer 
to  the  so-called  "pore-canals,"  which  are  common  in 
cuticular  structures  and  in  the  walls  of  many  cells 
which  bound  free  surfaces. 


196    THE  MORPHOLOGY  OF  THE  COMMON  CRAYFISH. 

The  whole  exoskeleton  of  the  crayfish  is,  in  fact, 
produced  by  the  cells  which  underlie  it,  either  by  tho 
exudation  of  a  chitinous  substance,  which  subsequently 
hardens,  from  them;  or,  as  is  more  probable,  by  the 
chemical  metamorphosis  of  a  superficial  zone  of  the 
bodies  of  the  cells  into  chitin.  However  this  may  be, 
the  cuticular  products  of  adjacent  cells  at  first  form  a 
simple,  continuous,  thin  pellicle.  A  continuation  of  the 
process  by  which  it  was  originated  increases  the  thick- 
ness of  the  cuticle ;  but  the  material  thus  added  to  the 
inner  surface  of  the  latter  is  not  always  of  the  same 
nature,  but  is  alternately  denser  and  softer.  The  denser 
material  gives  rise  to  the  tough  laminae,  the  softer  to 
the  intermediate  transparent  substance.  But  the  quan- 
tity of  the  latter  is  at  first  very  small,  whence  the  more 
external  laminaa  are  in  close  apposition.  Subsequently 
the  quantity  of  the  intermediate  substance  increases,  and 
gives  rise  to  the  thick  stratification  of  the  middle  region, 
while  it  remains  insignificant  in  the  inner  region  of  the 
exoskeleton. 

The  cuticular  structures  of  the  crayfish  differ  from 
the  nails,  hairs,  hoofs,  and  similar  hard  parts  of  the 
higher  animals,  insomuch  as  the  latter  consist  of  aggrega- 
tions of  cells,  the  bodies  of  which  have  been  metamor- 
phosed into  horny  matter.  The  cuticle,  with  all  its 
dependencies,  on  the  contrary,  though  no  less  dependent 
on  cells  for  its  existence,  is  a  derivative  product,  the 
formation  of  which  does  not  involve  the  complete  meta- 


CUTICULAH  TISSUE.  197 

morphosis  and  consequent  destruction  of  the  cells  to 
which  it  owes  its  origin. 

The  calcareous  salts  by  which  the  calcined  exoskeleton 
is  hardened  can  only  be  supplied  by  the  infiltration  of  a 
fluid  in  which  they  are  dissolved  from  the  blood ;  while 
the  distinctive  structural  characters  of  the  epiostracum, 
the  ectostracum,  and  the  endostracum,  are  the  results  of 
a  process  of  metamorphosis  which  goes  on  pari  passu  with 
this  infiltration.  To  what  extent  this  metamorphosis  is 
a  properly  vital  process  ;  and  to  what  extent  it  is  explic- 
able by  the  ordinary  physical  and  chemical  properties  of 
the  animal  membrane  on  the  one  hand,  and  the  mineral 
salts  on  the  other,  is  a  curious,  and  at  present,  un- 
solved problem. 

The  outer  surface  of  the  cuticle  is  rarely  smooth. 
Generally  it  is  more  or  less  obviously  ridged  or  tuhercu- 
lated ;  and,  in  addition,  presents  coarser  or  finer  hair- 
like  processes  which  exhibit  every  gradation  from  a  fine 
microscopic  down  to  stout  spines.  As  these  processes, 
though  so  similar  to  hairs  in  general  appearance,  are 
essentially  different  from  the  structures  known  as  hairs 
in  the  higher  animals,  it  is  better  to  speak  of  them  as 
teta. 

These  setae  (fig.  56,  F)  are  sometimes  short,  slender, 
conical  filaments,  the  surface  of  which  is  quite  smooth ; 
sometimes  the  surface  is  produced  into  minute  serra- 
tions, or  scale-like  prominences,  disposed  in  two  or  more 
series ;  in  other  setae,  the  axis  gives  off  slender  lateral 


198        THE  MORPHOLOGY  OF   THE   COMMON   CRAYFISH. 

branches ;  and  in  the  most  complicated  form  the  branches 
are  ornamented  with  lateral  branchlets.  For  a  certain 
distance  from  the  base  of  the  seta,  its  surface  is  usually 
smooth,  even  when  the  rest  of  its  extent  is  ornamented 
with  scales  or  branches.  Moreover,  the  basal  part  of  the 
seta  is  marked  off  from  its  apical  moiety  by  a  sort  of 
joint  which  is  indicated  by  a  slight  constriction,  or  by  a 
peculiarity  in  the  structure  of  the  cuticula  at  this  point. 
A  seta  almost  always  takes  its  origin  from  the  bottom  of 
a  depression  or  pit  of  the  layer  of  cuticle,  from  which  it  is 
developed,  and  at  its  junction  with  the  latter  it  is  generally 
thin  and  flexible,  so  that  the  seta  moves  easily  in  its 
socket.  Each  seta  contains  a  cavity,  the  boundaries  of 
which  generally  follow  the  outer  contours  of  the  seta.  In 
a  good  many  of  the  setae,  however,  the  parietes,  near  the 
base  of  the  seta,  are  thickened  in  such  a  manner  as 
almost,  or  completely,  to  obliterate  the  central  cavity. 
However  thick  the  cuticle  may  be  at  the  point  from 
which  the  setae  take  their  origin,  it  is  always  traversed 
by  a  funnel-shaped  canal  (fig.  56,  B,  d),  which  usually 
expands  beneath  the  base  of  the  seta.  Through  this 
canal  the  subjacent  ectoderm  extends  up  to  the  base  of 
the  seta,  and  can  even  be  traced  for  some  distance  into 
its  interior. 

It  has  already  been  mentioned  that  the  apodemata  and 
the  tendons  of  the  muscles  are  infoldings  of  the  cuticle, 
embraced  and  secreted  by  corresponding  involutions  of 
the  ectoderm. 


MOErHOLOGICAL   SUMMARY.  109 

Thus  the  body  of  the  cra^yfish  is  resolvable,  in  the  first 
place,  into  a  repetition  of  similar  segments,  the  metameres, 
each  of  which  consists  of  a  somite  and  two  appendages ; 
Hie  metameres  are  built  up  out  of  a  few  simple  tissues; 
and,  finally,  the  tissues  are  either  aggregates  of  more  or 
less  modified  nucleated  cells,  or  are  products  of  such  cells. 
Hence,  in  ultimate  morphological  analysis,  the  crayfish 
is  a  multiple  of  the  histological  unit,  the  nucleated  cell. 

What  is  true  of  the  crayfish,  is  certainly  true  of  all 
animals,  above  the  very  lowest.  And  it  cannot  yet  be  con- 
sidered certain  that  the  generalization  fails  to  held  good 
even  of  the  simplest  manifestations  of  animal  life ;  since 
recent  investigations  have  demonstrated  the  presence  of 
a  nucleus  in  organisms  in  which  it  had  hitherto  appeared 
to  be  absent. 

However  this  may  be,  there  is  no  doubt  that  in  the 
case  of  man  and  of  all  vertebrated  animals,  in  that 
of  all  arthropods,  mollusks,  echinoderms,  worms,  and 
inferior  organisms  down  to  the  very  lowest  sponges,  the 
process  of  morphological  analysis  yields  the  same  result 
as  in  the  case  of  the  crayfish.  The  body  is  built  up  of 
tissues,  and  the  tissues  are  either  obviously  composed  of 
nucleated  cells  ;  or,  from  the  presence  of  nuclei,  they 
may  be  assumed  to  be  the  results  of  the  metamorphosis 
of  such  cells  ;  or  they  are  cuticular  structures. 

The  essential  character  of  the  nucleated  cell  is  that  it 
consists  of  a  protoplasmic  substance,  one  part  of  uhich 
differs  somewhat  in  its  physical  and  chemical  characters 


-00    THE  MORPHOLOGY  OF  THE  COMMON  CRAYFISH 

from  the  rest,  and  constitutes  the  nucleus.  What  part 
the  nucleus  plaj^s  in  relation  to  the  functions,  or  vital 
activities,  of  the  cell  is  as  yet  unknown  ;  but  that  it  is 
the  seat  of  operations  of  a  different  character  from  those 
which  go  on  in  the  body  of  the  cell  is  clear  enough, 
I^or,  as  we  have  seen,  however  different  the  several 
tissues  may  be,  the  nuclei  which  they  contain  are  very 
much  alike ;  whence  it  follows,  that  if  all  these  tissues 
were  primitively  composed  of  simple  nucleated  cells,  it 
must  be  the  bodies  of  the  cells  which  have  undergone 
metamorphosis,  while  the  nuclei  have  remained  rela- 
tively unchanged. 

On  the  other  hand,  when  cells  multiply,  as  they  do 
in  all  growing  parts,  by  the  division  of  one  cell  into  two, 
the  signs  of  the  process  of  internal  change  which  ends 
in  fission  are  apparent  in  the  nucleus  before  they  are 
manifest  in  the  body  of  the  cell;  and,  commonly,  the 
division  of  the  former  precedes  that  of  the  latter.  Thus 
a  single  cell  body  may  possess  two  nuclei,  and  may  be- 
come divided  into  two  cells  by  the  subsequent  aggrega- 
tion of  the  two  moieties  of  its  protoplasmic  substance 
round  each  of  them,  &s  a  centre. 

In  some  cases,  very  singular  structural  changes  take 
place  in  the  nuclei  in  the  course  of  the  process  of  cell- 
division.  The  granular  or  fibrillar  contents  of  the 
nucleus,  the  wall  of  which  becomes  less  distinct,  arrange 
themselves  in  the  form  of  a  spindle  or  double  cone, 
formed  of  extremely  delicate  filaments  ;  and  in  the  plane 


THE   DIVISION  OF   NUCLEI.  201 

of  the  base  of  the  double  cone  the  filaments  present  knots 
or  thickenings,  just  as  if  they  were  so  many  threads  with 
a  bead  in  the  middle  of  each.  When  the  nuclear  spindle 
is  viewed  sideways,  these  beads  or  thickenings  give  rise 
to  the  appearance  of  a  disk  traversing  the  centre  of  the 
spindle.  Soon  each  bead  separates  into  two,  and  these 
move  away  from  one  another,  but  remain  connected  by  a 
fine  filament.  Thus  the  structure  which  had  the  form  of 
a  double  cone,  with  a  disk  in  the  middle,  assumes  that  of 
a  short  cylinder,  with  a  disk  and  a  cone  at  each  end.  But 
as  the  distance  between  the  two  disks  increases,  the 
uniting  filaments  lose  their  parallelism,  converge  in  the 
middle,  and  finally  separate,  so  that  two  separate  double 
cones  are  developed  in  place  of  the  single  one.  Along 
with  these  changes  in  the  nucleus,  others  occur  in  the 
protoplasm  of  the  cell  body,  and  its  parts  commonly  dis- 
play a  tendency  to  arrange  themselves  in  radii  from  the 
extremities  of  the  cones  as  a  centre ;  while,  as  the  separa- 
tion of  the  two  secondary  nuclear  spindles  becomes  com- 
plete, the  cell  body  gradually  splits  from  the  periphery 
inwards,  in  a  direction  at  right  angles  to  the  common 
axis  of  the  spindles  and  between  their  apices.  Thus 
two  cells  are  formed,  where,  previously,  only  one  existed ; 
and  the  nuclear  spindles  of  each  soon  revert  to  the 
globular  form  and  confused  arrangement  of  the  con- 
tents, characteristic  of  nuclei  in  their  ordinary  state* 
The  formation  of  these  nuclear  spindles  is  very  beau* 
tifully  seen  in  the  epithelial  cells  of  the  testis  of  the 


202         THE   MORPHOLOGY   OF   THE   COMMON   CRAYFISH. 

crayfish  (fig.  33,  p.  132) ;  but  I  have  not  been  able  to  find 
distinct  evidence  of  it  elsewhere  in  this  animal;  and 
although  the  process  has  now  been  proved  to  take  place 
in  all  the  divisions  of  the  animal  kingdom,  it  would  seem 
that  nuclei  may,  and  largely  do,  undergo  division,  without 
becoming  converted  into  spindles. 

The  most  cursory  examination  of  any  of  the  higher 
plants  shows  that  the  vegetable,  like  the  animal  body, 
is  made  up  of  various  kinds  of  tissues,  such  as  pith, 
woody  fibre,  spiral  vessels,  ducts,  and  so  on.  But  even 
the  most  modified  forms  of  vegetable  tissue  depart  so 
little  from  the  type  of  the  simple  cell,  that  the  reduction 
of  them  all  to  that  common  type  is  suggested  still  more 
strongly  than  in  the  case  of  the  animal  fabric.  And 
thus  the  nucleated  cell  appears  to  be  the  morphological 
unit  of  the  plant  no  less  than  of  the  animal.  Moreover, 
recent  inquiry  has  shown  that  in  the  course  of  the 
multiplication  of  vegetable  cells  by  division,  the  nuclear 
spindles  may  appear  and  run  through  all  their  remark- 
able changes  by  stages  precisely  similar  to  those  which 
occur  in  animals. 

The  question  of  the  universal  presence  of  nuclei  in 
cells  ma}T  be  left  open  in  the  case  of  Plants,  as  in  that 
of  Animals  ;  but,  speaking  general!}7,  it  may  justly  be 
affirmed  that  the  nucleated  cell  is  the  morphological 
foundation  of  both  divisions  of  the  living  world;  and 
the  great  generalisation  of  Schleiden  and  Schwann, 
that  there  is  a  fundamental  agreement  in  structure  and 


COMPARATIVE  MORPHOLOGY.  20o 

development  between  plants  and  animals,  has,  in  sub- 
stance, been  merely  confirmed  and  illustrated  by  the 
labours  of  the  half  century  which  has  elapsed  since  its 
promulgation. 

Not  only  is  it  true  that  the  minute  structure  of  the 
crayfish  is,  in  principle,  the  same  as  that  of  any  other 
Miiimal,  or  of  any  plant,  however  different  it  may  be  in 
detail ;  but,  in'  all  animals  (save  some  exceptional  forms) 
above  the  lowest,  the  body  is  similarly  composed  of 
three  layers,  ectoderm,  mesoderm,  and  endoderm,  dis- 
posed around  a  central  alimentary  cavity.  The  ectoderm 
and  the  endoderm  always  retain  their  epithelial  character ; 
while  the  mesoderm,  which  is  insignificant  in  the  lower 
organisms,  becomes,  in  the  higher,  far  more  complicated 
even  than  it  is  in  the  crayfish. 

Moreover,  in  the  whole  of  the  Arthropoda,  and  the 
whole  of  the  Vertebrata,  to  say  nothing  of  other  groups 
of  animals,  the  body,  as  in  the  crayfish,  is  susceptible 
of  distinction  into  a  series  of  more  or  less  numerous 
segments,  composed  of  homologous  parts.  In  each 
segment  these  parts  are  modified  according  to  physio- 
logical requirements  ;  and  by  the  coalescence,  segrega- 
tion, and  change  of  relative  size  and  position  of  the 
segments,  well  characterized  regions  of  the  body  are 
marked  out.  And  it  is  remarkable  that  precisely  the 
same  principles  are  illustrated  by  the  morphology  of 
plints.  A  flower  with  its  whorls  of  sepals,  petals, 
stamens  and  carpels  has  the  same  relation  to  a  stem 


204         THE  MORPHOLOGY  OF  THE  COMMON  CRAYFISH. 

with  its  whorls  of  leaves,  as  a  crayfish's  head  has  to  its 
abdomen,  or  a  dog's  skull  to  its  thorax. 

It  may  be  objected,  however,  that  the  morphological 
generalisations  which  have  now  been  reached,  are  to  a 
considerable  extent  of  a  speculative  character ;  and  that,  in 
the  case  of  our  crayfish,  the  facts  warrant  no  more  than 
the  assertion  that  the  structure  of  that  animal  may  be 
consistently  interpreted,  on  the  supposition  that  the  body 
is  made  up  of  homologous  somites  and  appendages,  and 
that  the  tissues  are  the  result  of  the  modification  of 
homologous  histological  elements  or  cells ;  and  the  ob- 
jection is  perfectly  valid. 

There  can  be  no  doubt  that  blood  corpuscles,  liver 
cells,  and  ova  are  all  nucleated  cells ;  nor  any  that  the 
third,  fourth,  and  fifth  somites  of  the  abdomen  are  con- 
structed upon  the  same  plan ;  for  these  propositions  are 
mere  statements  of  the  anatomical  facts.  But  when,  from 
the  presence  of  nuclei  in  connective  tissue  and  muscles, 
we  conclude  that  these  tissues  are  composed  of  modified 
cells ;  or  when  we  say  that  the  ambulatory  limbs  of  the 
thorax  are  of  the  same  type  as  the  abdominal  limbs,  the 
exopodite  being  suppressed,  the  statement,  as  the  evi- 
dence stands  at  present,  is  no  more  than  a  convenient 
way  of  interpreting  the  facts.  The  question  remains, 
has  the  muscle  actually  been  formed  out  of  nucleated 
cells  ?  Has  the  ambulatory  limb  ever  possessed  an 
exopodite,  and  lost  it  ? 


DEVELOPMENT — YELK  DIVISION.  205 

The  answer  to  these  questions  is  to  be  sought  in  the 
facts  of  individual  and  ancestral  development. 

An  animal  not  only  is,  but  becomes ;  the  crayfish  is  the 
product  oi  an  egg,  in  which  not  a  single  structure  visible 
in  the  adult  animal  exists  :  in  that  egg  the  different  tissues 
and  organs  make  their  appearance  by  a  gradual  process  of 
evolution ;  and  the  study  of  this  process  can  alone  tell 
us  whether  the  unity  of  composition  suggested  by  the 
comparison  of  adult  structures,  is  borne  out  by  the  facts 
of  their  development  in  the  individual  or  not.  The 
hypothesis  that  the  body  of  the  crayfish  is  made  up  of  a 
series  of  homologous  somites  and  appendages,  and  that 
all  the  tissues  are  composed  of  nucleated  cells,  might  be 
only  a  permissible,  because  a  useful,  mode  of  colligating 
the  facts  of  anatomy.  The  investigation  of  the  actual 
manner  in  which  the  evolution  of  the  body  of  the  crayfish 
has  been  effected,  is  the  only  means  of  ascertaining 
whether  it  is  anything  more.  And,  in  this  sense,  deve- 
lopment is  the  criterion  of  all  morphological  speculations. 

The  first  obvious  change  which  takes  place  in  an  im- 
pregnated ovum  is  the  breaking  up  of  the  yelk  into 
smaller  portions,  each  of  which  is  provided  with  a  nucleus, 
*ml  is  termed  a  blastojnere.  In  a  general  morphological 
sense,  a  blastomere  is  a  nucleated  cell,  and  differs  from 
an  ordinary  cell  only  in  size,  and  in  the  usual,  though  by 
no  means  invariable,  abundance  of  granular  contents ; 
and  blastomeres  insensibly  pass  into  ordinary  cells,  as 


200        THE  MORPHOLOGY  OF  THE   COMMON   CRAYFISH. 

the  process  of  division  of  the  yelk  into  smaller  and 
smaller  portions  goes  on. 

In  a  great  many  animals,  the  splitting-up  into  blasto- 
meres  is  effected  in  such  a  manner  that  the  yelk  is,  at 
first,  divided  into  equal,  or  nearly  equal,  masses ;  that 
each  of  these  again  divides  into  two  ;  and  that  the  number 
of  blastomeres  thus  increases  in  geometrical  progression 
until  the  entire  yelk  is  converted  into  a  mulberry-like 
body,  termed  a  morula,  made  up  of  a  great  number 
of  small  blastomeres  or  nucleated  cells.  The  whole 
organism  is  subsequently  built  up  by  the  multiplication, 
the  change  of  position,  and  the  metamorphosis  of  these 
products  of  yelk  division. 

In  such  a  case  as  this,  yelk  division  is  said  to  be 
complete.  An  unessential  modification  of  complete  yelk 
division  is  seen  when,  at  an  early  period,  the  blastomeres 
produced  by  division  are  of  unequal  sizes ;  or  when  they 
become  unequal  in  consequence  of  division  taking  place 
much  more  rapidly  in  one  set  than  in  another. 

In  many  animals,  especially  those  which  have  large 
ova,  the  inequality  of  division  is  pushed  so  far  that  only 
a  portion  of  the  yelk  is  affected  by  the  process  of  fission, 
while  the  rest  serves  merely  as  food-yelk,  for  nutriment 
to  the  blastomeres  thus  produced.  Over  a  greater  or 
less  extent  of  the  surface  of  the  egg,  the  protoplasmic 
substance  of  the  yelk  segregates  itself  from  the  rest, 
and,  constituting  a  germinal  layer,  breaks  up  into  the 
blastomeres,  which  multiply  at  the  expense  of  the  food- 


THE  FORMATION  OF  A  BLASTODERM.       207 

yelk,  and  fabricate  the  body  of  the  embryo.  This  process 
is  termed  partial  or  incomplete  yelk  division. 

The  crayfish  is  one  of  those  animals  in  the  egg  of 
which  the  yelk  undergoes  partial  division.  The  first 
steps  of  the  process  have  not  yet  been  thoroughly  worked 
out,  but  their  result  is  seen  in  ova  which  have  been  but 
a  short  time  laid  (fig.  57,  A).  In  such  eggs,  the  great 
mass  of  the  substance  of  the  vitellus  is  destined  to  play 
the  part  of  food-yelk  ;  and  it  is  disposed  in  conical 
masses,  which  radiate  from  a  central  spheroidal  portion 
to  the  periphery  of  the  yelk  (17).  Corresponding  with  the 
base  of  each  cone,  there  is  a  clear  protoplasmic  plate, 
which  contains  a  nucleus  ;  and  as  these  bodies  are  all 
in  contact  by  their  edges,  they  form  a  complete,  though 
thin,  investment  to  the  food-yelk.  This  is  termed  the 
blastoderm  (bl). 

Each  nucleated  protoplasmic  plate  adheres  firmly  to 
the  corresponding  cone  of  granular  food-yelk,  and,  in  all 
probability,-  the  two  together  represent  a  blastornere ; 
but,  as  the  cones  only  indirectly  subserve  the  growth  of 
the  embryo,  while  the  nucleated  peripheral  plates  form 
an  independent  spherical  sac,  out  of  which  the  body  of 
the  young  crayfish  is  gradually  fashioned,  it  will  be  con- 
venient to  deal  with  the  latter  separately. 

Thus,  at  this  period,  the  body  of  the  developing  crayfish 
is  nothing  but  a  spherical  bag,  the  thin  walls  of  which  are 
composed  of  a  single  layer  of  nucleated  cells,  while  its 
cavity  is  filled  with  food-yelk.  The  first  modification 


Fro,  yJ.—Astacvs  fluviatilis. — Diagrammatic  sections  of  embryos  ;  partly  after  Reichenbaeh,  partly 
wnginal  ( x  20).  A.  An  ovum  in  which  the  blastoderm  is 'just  formed.  B.  An  ovum  in  which 
the  invagination  of  the  blastoderm  to  constitute  the  hypoblast  or  rudiment  of  the  mid-gut  has 
taken  place  (This  nearly  answers  to  the  stage  represented  in  fig.  58,  A.}  C.  A  longitudinal 
section  of  an  ovum,  in  which  the  rudiments  ot  the  abdomen,  of  the  hind-gut,  and  of  the  fore- 
gut  have  appeared.  (This  nearly  answers  to  the  stage  represented  in  fi<r.  53,  E.)  D.  A  similar 
section  of  an  embryo  in  nearly  the  same  stage  of  development  as  that  represented  in  C,  fig.  59. 
E.  An  embryo  just  hatched,  in  longitudinal  section  ;  a,  anus  ;  bl,  blastoderm  ;  ftp,  blasto- 
pore  ;  e,  eye  ;  ep.b. ,  epiblast ;  fg,  fore-gut ; /f/1,  its  oesophageal,  and /(/2,  its  gastric  portion; 
h,  heart;  kg,  hind-gut;  r»,  mouth;  mg,  hypoblast,  archenteron,  or  mid-gut;  v,  yelk.  The 
dotted  portions  in  D  and  E  represent  the  nervous  system. 


THE   ARCHENTERJC   INVAGINATION.  209 

which  is  effected  in  the  vesicular  blastoderm  manifests 
itself  on  that  face  of  it  which  is  turned  towards  the  pedicle 
of  the  egg.  Here  the  la}rer  of  cells  becomes  thickened 
throughout  an  oval  area  about  l-25th  of  an  inch  in 
diameter.  Hence,  when  the  egg  is  viewed  by  reflected 
light,  a  whitish  patch  of  corresponding  form  and  size 
appears  in  this  region.  This  may  be  termed  the  ger- 
minal disk.  Its  long  axis  corresponds  with  that  of  the 
future  crayfish. 

Next,  a  depression  (fig.  58,  A,  bp)  appears  in  the  hinder 
third  of  the  germinal  disk,  in  consequence  of  this  part 
of  the  blastoderm  growing  inwards,  and  thus  giving  rise 
to  a  small  wide-mouthed  pouch,  which  projects  into  the 
food-yelk  with  which  the  cavity  of  the  blastoderm  is 
filled  (fig.  57,  B,  ing).  As  this  infolding,  or  invagination 
of  the  blastoderm,  goes  on,  the  pouch  thus  produced 
increases,  while  its  external  opening,  termed  the  blasto- 
pore  (fig.  57,  B,  and  58,  A — E,^fcp),  diminishes  in  size. 
Thus  the  body  of  the  embryo  crayfish,  from  being  a 
simple  bag  becomes  a  double  bag,  such  as  might  be 
produced  by  pushing  in  the  wall  of  an  incompletely 
distended  india-rubber  ball  with  the  finger.  And,  in 
this  case,  if  the  interior  of  the  bag  contained  porridge, 
the  latter  would  very  fairly  represent  the  food-yelk. 

By  this  invagination  a  most  important  step  has 
been  taken  in  the  development  of  the  crayfish.  For, 
though  the  pouch  is  nothing  but  an  ingrowth  ol  part  of 

the  blastoderm,  the  cells  of  which  its  wall  is  composed, 
15 


Flo.  58.— Astacus  ftuviatilis. — Surface  views  of  the  earlier  stages  in  tlie  development  of 
the  embryo,  from  the  appearance  of  the  blastopore  (A)  to  the  assumption  of  the 
nauplins  iorm  (F)  (after  Reichenbach,  x  about  23).  t'p,  blast  o]  ore  ,  <;  carapace  ;  fg, 
fore-gut  involution  ;  h,  heart  ;  hg,  hind-put  involution  ;  lb,  labnim  ;  mg,  medullary 
groove  :  o,  optic  pit ;  p,  endodermal  plug  partly  firing  ->\]i  the  blastopore  ;  pc,  pio- 
cephalic  j)roceKses  ;  ta,  abdominal  elevation ;  %,  antennules ;  3  antennae  k,  mau- 


EPIBLAST,   MESOBLAST,   AND   HYPOBLAST.  211 

henceforward  exhibit  different  tendencies  from  those 
which  are  possessed  by  the  rest  of  the  blastoderm .  In 
fact,  it  is  the  primitive  alimentary  apparatus  or  archen- 
teron,  and  its  wall  is  termed  the  hypoblast.  The  rest  of 
the  blastoderm,  on  the  contrary,  is  the  primitive  epider- 
mis, and  receives  the  name  of  epiblast.  If  the  food- 
yelk  were  away,  and  the  archenteron  enlarged  until  the 
hypoblast  came  in  contact  with  the  epiblast,  the  entire 
body  would  be  a  double-walled  sac,  containing  an  ali- 
mentary cavity,  with  a  single  external  aperture.  This  is 
the  gastrula  condition  of  the  embryo  ;  and  some  animals, 
such  as  the  common  fresh-water  polype,  are  little  more 
thnn  permanent  gastrulce. 

Although  the  gastrula  has  not  the  slightest  resem- 
blance to  a  crayfish,  yet,  as  soon  as  the  hypoblast  and 
the  epiblast  are  thus  differentiated,  the  foundations  of 
some  of  the  most  important  systems  of  organs  of  the 
future  crustacean  are  laid.  The  hypoblast  will  give  rise 
to  the  epithelial  lining  of  the  mid-gut ;  the  epiblast 
(which  answers  to  the  ectoderm  in  the  adult)  to  the 
epithelia  of  the  fore-gut  and  hind-gut,  to  the  epidermis, 
and  to  the  central  nervous  system. 

The  mesodermal  structures,  that  is  to  say  the  con- 
nective tissue,  the  muscles,  the  heart  and  vessels,  and 
the  reproductive  organs,  which  lie  between  the  ectoderm 
and  the  endoderm,  are  not  derived  directly  from  either 
the  epiblast  or  the  hypoblast,  but  have  a  #&#  si-independent 
origin,  Irom  a  mass  of  cells  which  first  makes  its  appear- 


212        THE  MORPHOLOGY   OP  THE  COMMON  CRAYFISH. 

ance  in  the  neighbourhood  of  the  blastopcre,  between  the 
hypoblast  and  the  epiblast,  though  they  are  probably 
derived  from  the  former.  From  this  region  they  gradu- 
ally spread,  first  over  the  sternal,  and  then  on  to  the 
tergal  aspect  of  the  embryo,  and  constitute  the  mesollast. 

Epiblast,  hypoblast,  and  mesoblast  are  at  first  alike 
constituted  of  nothing  but  nucleated  cells,  and  they  in- 
crease in  dimensions  by  the  continual  fission  and  growth 
of  these  cells.  The  several  layers  become  gradually 
modelled  into  the  organs  which  they  constitute,  before 
the  cells  undergo  any  notable  modification  into  tissues. 
A  limb,  for  example,  is,  at  first,  a  mere  cellular  out- 
growth, or  bud,  composed  of  an  outer  coat  of  epiblast 
with  an  inner  core  of  mesoblast ;  and  it  is  only  subse- 
quently that  its  component  cells  are  metamorphosed  into 
well-defined  epidermic  and  connective  tissues,  vessels  and 
muscles. 

The  embryo  crayfish  remains  only  a  short  while  in 
the  gastrula  stage,  as  the  blastopore  soon  closes  up,  and 
the  archenteron  takes  the  form  of  a  sac,  flattened  out 
between  the  epiblast  and  the  food-yelk,  with  which  its 
cells  are  in  close  contact  (fig.  57,  C  and  D).*  Indeed,  as 
development  proceeds,  the  cells  of  the  hypoblast  actually 
feed  upon  the  substance  of  the  food-yelk,  and  turn  it  to 
account  for  the  general  nutrition  of  the  body. 

*  Whether,  as  some  observers  state,  the  hypoblastic  cells  grow  over 
and  inclose  the  food-yelk  or  not,  is  a  question  that  may  be  left  open.  I 
have  not  been  able  to  satisfy  myself  of  this  fact. 


THE   FORE-GUT.  213 

The  sternal  area  of  the  embryo  gradually  enlarges 
until  it  occupies  one  hemisphere  of  the  yelk;  in  other 
words,  the  thickening  of  the  epiblast  gradually  extends 
ant  wards.  Just  in  front  of  the  blastopore,  as  it  closes, 
the  middle  of  the  epiblast  grows  out  into  a  rounded 
elevation  (fig.  58,  ta;  fig.  59,  ab),  which  rapidly  increases 
in  length,  and  at  the  same  time  turns  forwards.  This 
is  the  rudiment  of  the  whole  abdomen  of  the  crayfish. 
Further  forwards,  two  broad  and  elongated,  but  flatter 
thickenings  appear ;  one  on  each  side  of  the  middle  line 
(fig.  58,  p  c).  As  the  free  end  of  the  abdominal  papilla 
now  marks  the  hinder  extremity  of  the  embryo,  so  do 
these  two  elevations,  which  are  termed  the  procephalic 
lobes,  define  its  anterior  termination.  The  whole  sternal 
region  of  the  body  will  be  produced  by  the  elongation  of 
that  part  of  the  embryo  which  lies  between  these  two 
limits. 

A  narrow  longitudinal  groove-like  depression  appears 
on  the  surface  6f  the  epiblast,  in  the  middle  line,  between 
the  procephalic  lobes  and  the  base  of  the  abdominal 
papilla  (fig.  58,  C — F,  mg).  About  its  centre,  this  groove 
becomes  further  depressed  by  the  ingrowth  of  the  epi 
blast,  which  constitutes  its  floor,  and  gives  rise  to  a 
short  tubular  sac,  which  is  the  rudiment  of  the  whole  fore - 
cut  (fig.  57,  C,  and  fig.  58,  E,/#).  At  first,  this  epiblastic 
ingrowth  does  not  communicate  with  the  archenteron,  but, 
aftoi  a  while,  its  blind  end  combines  with  the  front  and 
lower  part  of  the  hypoblast,  and  an  opening  is  formed  by 


214         THE  MORPHOLOGY  OF  THE   COMMON  CRAYFISH. 

which  the  cavity  of  the  fore -gut  communicates  with  that 
of  the  mid-gut  (fig.  57,  E).  Thus  a  gullet  and  stomach, 
or  rather  the  parts  which  will  eventually  give  rise  to  all 
these,  are  constituted.  And  it  is  important  to  remark 
that,  in  comparison  with  the  mid-gut,  they  are,  at  first, 
very  small. 

In  the  same  way,  the  epiblast  covering  the  sternal  face 
of  the  abdominal  papilla  undergoes  invagination  and  is 
converted  into  a  narrow  tube  which  is  the  origin  of  the 
whole  hind-gut  (fig.  57,  C,  and  fig.  58,  E,  %).  This,  like 
the  fore-gut,  is  at  first  blind  ;  but  the  shut  front  end  soon 
applying  itself  to  the  hinder  wall  of  the  archenteric  sac, 
the  two  coalesce  and  open  into  one  another  (fig.  57,  E). 
Thus  the  complete  alimentary  canal,  consisting  of  a  very 
narrow,  tubular,  fore-  and  hind-gut,  derived  from  the 
epiblast,  and  a  wider  and  more  sac-like  mid-gut,  formed 
of  the  whole  hypoblast,  is  constituted. 

The  procephalic  lobes  become  more  convex ;  while, 
behind  them,  the  surface  of  the  epiblast  rises  into  six 
elevations  disposed  in  pairs,  one  on  each  side  of  the 
median  groove.  The  hindermost  of  these,  which  lie  at 
the  sides  of  the  mouth,  are  the  rudiments  of  the 
mandibles  (fig.  58,  E  and  F,  4 ) ;  the  other  two  become 
the  antennaB  (3)  and  the  antennules  (2],  while,  at  a  later 
period,  processes  of  the  procephalic  lobes  give  rise  to  the 
eyestalks. 

A  short  distance  behind  the  abdomen,  the  epiblast 
rises  into  a  transverse  ridge,  which  is  concave  forwards, 


THE  TRANSITORY   NAUPLIUS   STAGE.  215 

while  its  ends  are  prolonged  on  each  side  nearly  as  far 
as  the  mouth.  This  is  the  commencement  of  the  free 
edge  of  the  carapace  (fig.  58,  E  and  F,  and  fig.  59,  A,  c) 
—the  lateral  parts  of  which,  greatly  enlarging,  become 
tho  branchiostegites  (fig.  59,  D,  c). 

In  many  animals  allied  to  crayfish,  the  young,  when 
it  has  reached  a  stage  in  its  development,  which  answers 
to  this,  undergoes  rapid  changes  of  outward  form  and  of 
internal  structure,  without  making  any  essential  addition 
to  the  number  of  the  appendages.  The  appendages  which 
represent  the  antennules,  the  antenna,  and  the  mandibles 
elongate  and  become  oar-like  locomotive  organs ;  a 
single  median  eye  is  developed,  and  the  young  leaves  the 
egg  as  an  active  larva,  which  is  known  as  a  Nauplius. 
The  crayfish,  on  the  other  hand,  is  wholly  incapable  of 
an  independent  existence  at  this  stage,  and  continues  its 
embryonic  life  within  the  egg  case  ;  but  it  is  a  remark- 
able circumstance  that  the  cells  of  the  epiblast  secrete 
a  delicate  cuticula,  which  is  subsequently  shed.  It  is 
as  if  the  animal  symbolized  a  nauplius  condition  by 
the  development  of  this  cuticle,  as  the  foetal  whalebone 
whale  symbolizes  a  toothed  condition  by  developing  teeth 
which  are  subsequently  lost  and  never  perform  any 
function. 

In  fact,  in  the  crayfish,  the  nauplius  condition  is  soon 
left  behind.  The  sternal  disk  spreads  more  and  more 
over  the  yelk ;  as  the  region  between  the  mouth  and 
the  root  of  the  abdomen  elongates,  slight  transverse 


ab. 


14. 


FIG.  59.—  ^stacus  ftuviatiUs.—  Ventral  (A,  B,  C,  F)  and  lateral  (D,  E)  views  of  the 
embryo  in  successive  stages  of  development  (after  Kathke,  x  lo).  A  is  a  little  more 
advanced  than  the  embryo  represented  in  fig.  58,  F  :  D,  E,  and  F  are  views  of 
the  young  crayfish  when  nearly  ready  to  be  hatched  :  in  E,  the  carapace  is 
remoVed,  and  the  limbs  and  abdomen  are  spread  out.  1 — 14,  the  cephalic  and 
thoracic,  appendages;  all,  abdomen;  br,  brunelme ;  <\  carapace:  cp,  epipodite 
of  the  first  maxillipede ;  gg,  green  gland ;  h,  heart ;  Ib.  labrum  ;  Ir,  liver ;  ra, 
mandibular  muscles. 


THE   DEVELOPMENT   OF   THE   LIMBS.  217 

depressions  indicate  the  boundaries  of  the  posterior 
cephalic  and  the  thoracic  somites ;  and  pairs  of  eleva- 
tions, similar  to  the  rudiments  of  the  antennules  and 
antennae,  appear  upon  them  in  regular  order  from  before 
backwards  (fig.  59,  C). 

In  the  meanwhile,  the  extremity  of  the  abdomen 
flattens  out  and  takes  on  the  form  of  an  oval  plate, 
the  middle  of  the  posterior  margin  of  which  is  slightly 
truncated  or  notched ;  while,  finally,  transverse  constric- 
tions mark  off  six  segments,  the  somites  of  the  abdomen, 
in  front  ol  this.  Along  with  these  changes,  four  pairs 
of  tubercles  grow  out  from  the  sternal  faces  of  the  four 
middle  abdominal  somites,  and  constitute  the  rudiments 
of  the  four  middle  pairs  of  abdominal  appendages.  The 
first  abdominal  somite  exhibits  only  two  hardly  percept- 
ible elevations  in  place  of  the  appendages  of  the  others, 
while  the  sixth  seems,  at  first,  to  have  none.  The  ap- 
pendages of  the  sixth  somite,  however,  are  already  formed, 
though,  singularly  enough,  they  lie  beneath  the  cuticle 
of  the  telson  and  are  set  free  only  after  the  first 
ecdysis. 

The  rostrum  grows  out  between  the  procephalic  lobes ; 
it  remains  relatively  very  short  up  to  the  time  that  the 
young  crayfish  quits  the  egg,  and  is  directed  more  down- 
wards than  forwards.  The  lateral  portions  of  the  cara- 
pacial  ridge,  becoming  deeper,  are  converted  into  the 
branchiostegites,  and  the  cavities  which  they  overarch 
are  the  branchial  chambers.  The  transverse  portion  of 


218         THE   MORPHOLOGY   OF   THE   COMMON    CRAYFISH. 

the  ridge,  on  the  other  hand,  remains  relative!}7  short,  and 
constitutes  the  free  posterior  margin  of  the  carapace. 

As  these  changes  take  place,  the  abdomen  and  the 
sternal  region  of  the  thorax  are  constantly  enlarging  in 
proportion  to  the  rest  of  the  ovum ;  and  the  food-yelk 
which  lies  in  the  cephalothorax  is,  pari  passu,  being 
diminished.  Hence  the  cephalothorax  constantly  becomes 
relatively  smaller  and  the  tergal  aspect  of  the  carapace 
less  spherical;  although,  even  when  the  young  crayfish 
is  ready  to  be  hatched,  the  difference  between  it  and  the 
adult  in  the  form  of  the  cephalo thoracic  region,  and  in  the 
size  of  the  latter  relatively  to  the  abdomen,  is  very  marked. 

The  simple  bud-like  outgrowths  of  the  somites,  in 
which  all  the  appendages  take  their  origin,  are  rapidly 
metamorphosed.  The  eyestalks  (fig.  59,  T)  soon  attain 
a  considerable  relative  size.  The  extremities  of  the 
antennules  (2)  and  of  the  antennae  (8)  become  bifurcated ; 
and  the  two  divisions  of  the  antennule  remain  broad, 
thick,  and  of  nearly  the  same  size  up  to  birth.  On  the 
other  hand,  the  inner  or  endopoditic  division  of  the 
antenna  becomes  immensely  lengthened,  and  at  the  same 
time  annulated,  while  the  outer  or  exopoditic  division 
remains  relatively  short,  and  acquires  its  characteristic 
scale-like  form. 

The  labrum  (lb)  arises  as  a  prolongation  of  the  middle 
sternal  region  in  front  of  the  mouth,  while  the  bilobed 
metastoma  is  an  outgrowth  of  the  sternal  region  be- 
hind it. 


THE   NEWLY-HATCHED   CRAYFISH.  219 

The  posterior  cephalic  and  the  thoracic  appendages 
(5 — 24)  elongate  and  gradually  approach  the  form  which 
they  possess  in  the  adult.  I  have  not  been  able  to 
discover,  at  any  period  of  development,  an  outer  division 
or  exopodite  in  any  of  the  five  posterior  thoracic  limbs. 
And  this  is  a  very  remarkable  circumstance,  inasmuch 
as  such  an  exopodite  exists  in  the  closely  allied  lobster 
in  the  larval  state  ;  and,  in  many  of  the  shrimp  and 
prawn-like  allies  of  the  crayfish^  a  complete  or  rudi- 
mentary exopodite  is  found  in  these  limbs,  even  in  the 
adult  condition. 

When  the  crayfish  is  hatched  (fig.  60)  it  differs  from  the 
adult  in  many  ways — not  only  is  the  cephalothorax  more 
convex  and  larger  in  proportion  to  the  abdomen  ;  but  the 
rostrum  is  short  and  bent  down  between  the  eyes.  The 
sterna  of  the  thorax  are  wider  relatively,  and  hence  there 
is  a  greater  interval  between  the  bases  of  the  legs  than  in 
the  adult.  The  proportion  of  the  limbs  to  one  another 
and  to  the  body  are  nearly  the  same  as  in  the  adult,  but 
the  chelae  of  the  forceps  are  more  slender.  The  tips  of 
the  chelae  are  all  strongly  incurved  (fig.  8,  B,  p.  41),  and  the 
dactylopodites  of  the  two  posterior  thoracic  limbs  are  hook- 
like.  The  appendages  of  the  first  abdominal  somite  are  un- 
developed, and  those  of  the  last  are  inclosed  within  the 
telson,  which  is,  as  has  already  been  said,  of  a  broad  oval 
form,  usually  notched  in  the  middle  of  its  hinder  margin, 
and  devoid  of  any  indication  of  transverse  division.  Its 
margins  are  produced  into  a  single  series  of  short  conical 


220         THE  MOKPHOLOGY   OF   THE   COMMON   CRAYFISH. 

processes,  and  the  disposition  of  the  vascular  canals  in  its 
interior  gives  it  the  appearance  of  being  radially  striated. 
The  set®,  so  abundant  in  the  adult,  are  very  scanty  in 
the  newly  hatched  young;  and  the  great  majority  of  those 
which  exist  are  simple  conical  prolongations  of  the  un- 


FIG.  60. — Astacus  flnviatilis.— Newly-hatched  young  (  x  6). 

calcified  cuticle,  the  bases  of  which  are  not  sunk  in  pits 
and  which  are  devoid  of  lateral  scales  or  processes. 

The  young  animals  are  firmly  attached  to  the  ab- 
dominal appendages  of  the  parent  in  the  manner  already 
described.  They  are  very  sluggish,  though  they  move 
when  touched ;  and  at  this  period  they  do  not  feed,  but 


THE   EVOLUTION   OF  THE   INDIVIDUAL.  221 

are  nourished  by  the  food-yelk,  of  which  a  considerable 
store  still  remains  in  the  cephalothorax. 

I  imagine  that  they  are  set  free  during  the  first  ecdysis, 
and  that  the  appendages  of  the  sixth  abdominal  somite 
are  at  that  time  expanded,  but  nothing  is  definitely  known 
at  present  of  these  changes. 

The  foregoing  sketch  of  the  general  nature  of  the 
changes  which  take  place  in  the  egg  of  the  crayfish 
suffice  to  show  that  its  development  is,  in  the  strictest 
sense  of  the  word,  a  process  of  evolution.  The  egg  is 
a  relatively  homogeneous  mass  of  living  protoplasmic 
matter,  containing  much  nutritive  material ;  and  the 
development  of  the  crayfish  means  the  gradual  conver- 
sion of  this  comparatively  simple  body  into  an  organism 
of  great  complexity.  The  yelk  becomes  differentiated 
into  formative  and  nutritive  portions.  The  formative 
portion  is  subdivided  into  histological  units :  these 
arrange  themselves  into  a  blastoderinic  vesicle ;  the  blas- 
toderm becomes  differentiated  into  epiblast,  hypoblast, 
and  mesoblast ;  and  the  simple  vesicle  assumes  the  gas- 
trula  condition.  The  layers  of  the  gastrula  shape  them- 
selves into  the  body  of  the  crayfish  and  its  appendages, 
while  along  with  this,  the  cells  of  which  all  the  parts 
are  built,  become  metamorphosed  into  tissues,  each  with 
its  characteristic  properties.  And  all  these  wonderful 
changes  are  the  necessary  consequences  of  the  interaction 
of  the  molecular  forces  resident  in  the  substance  of  the 


222         THE  MORPHOLOGY   OF  THE  COMMON   CRAYFISH. 

impregnated  ovum,  with  the  conditions  to  which  it  is 
exposed  ;  just  as  the  forms  evolved  from  a  crystallising 
fluid  are  dependent  upon  the  chemical  composition  of 
the  dissolved  matter  and  the  influence  of  surrounding 
conditions. 

Without  entering  into  details  which  lie  heyond  the 
scope  of  the  present  work,  something  must  be  said  re- 
specting the  manner  in  which  the  complicated  internal 
organisation  of  the  crayfish  is  evolved  from  the  cellular 
double  sac  of  the  gastrula  stage. 

It  has  been  seen  that  the  fore-gut  is  at  first  an  insig- 
nificant tubular  involution  of  the  epiblast  in  the  region 
of  the  mouth.  It  is,  in  fact,  a  part  of  the  epiblast  turned 
inwards,  and  the  cells  of  which  it  is  composed  secrete  a 
thin  cuticular  layer,  as  do  those  of  the  rest  of  the  epi- 
blast, which  gives  rise  to  the  ectodermal  or  epidermic 
part  of  the  integument.  As  the  embryo  grows,  the  fore- 
gut  enlarges  much  faster  than  the  mid-gut,  increasing 
in  height  and  from  before  backwards,  while  its  side-walls 
remain  parallel,  and  are  separated  by  only  a  narrow 
cavity.  At  length,  it  takes  on  the  shape  of  a  triangular 
bag  (fig.  57,  D,  /#),  attached  by  its  narrow  end  around 
the  mouth  and  immersed  in  the  food-yelk,  which  it 
gradually  divides  into  two  lobes,  one  on  the  right  and  one 
on  the  left  side.  At  the  same  time  a  vertical  plate  of 
mesoblastic  tissue,  from  which  the  great  anterior  and 
posterior  muscles  are  eventually  developed,  connects  it 
with  the  roof  and  with  the  front  wall  of  the  carapace, 


ORIGIN  OF  THE  INTERNAL  ORGANS.  223 

Becoming  constricted  in  the  middle,  the  fore-gut  next 
appears  to  consist  of  two  dilatations  of  about  equal  size, 
connected  by  a  narrower  passage  (fig.  57,  E,  fgl9  fg*). 
The  front  dilatation  becomes  the  oesophagus  and  the 
cardiac  division  of  the  stomach ;  the  hinder  one,  the 
pyloric  division.  At  the  sides  of  the  front  end  of  the 
cardiac  division  two  small  pouches  are  formed  shortly 
after  birth ;  in  each  of  these  a  thick  laminated  deposit 
of  chitin  takes  place,  and  constitutes  a  minute  crab's-eye 
or  gastrolith,  which  has  the  same  structure  as  in  the 
adult,  and  is  largely  calcined.  This  fact  is  the  more 
remarkable  as,  at  this  time,  the  exoskeleton  contains  very 
little  calcareous  deposit.  In  the  position  of  the  gastric 
teeth,  folds  of  the  cellular  wall  of  corresponding  shape 
are  formed,  and  the  chitinous  cuticle  of  which  the  teeth 
are  composed  is,  as  it  were,  modelled  upon  them. 

The  hind-gut  occupies  the  whole  length  of  the  abdo- 
men, and  its  cells  early  arrange  themselves  into  six 
ridges,  and  secrete  a  cuticular  layer. 

The  mid-gut,  or  hypoblastic  sac,  very  soon  gives  off 
numerous  small  prolongations  on  each  side  of  its  hinder 
extremity,  and  these  are  converted  into  the  caeca  of  the 
liver  (fig.  57,  E,  mg}.  The  cells  of  its  tergal  wall  are  in 
close  contact  with  the  adjacent  masses  of  food-yelk ;  and 
it  is  probable  that  the  gradual  absorption  of  the  food- 
yelk  is  chiefly  effected  by  these  cells.  At  birth,  however, 
the  lateral  lobes  of  the  food-yelk  are  still  large,  and 
occupy  the  space  left  between  the  stomach  and  liver 


224         THE  MORPHOLOGY    OF  THE   COMMON   CRAYFISH. 

on  the  one  hand,  and  the  cephalic  integument  on  the 
other. 

The  mesoblastic  cells  give  rise  to  the  layer  of  con- 
nective tissue  which  forms  the  deeper  portion  of  the 
integument,  and  to  that  which  invests  the  alimentary 
canal ;  to  all  the  muscles ;  and  to  the  heart,  the  vessels, 
and  the  corpuscles  of  the  blood.  The  heart  appears 
very  early  as  a  solid  mass  of  mesoblastic  cells  in  the 
tergal  region  of  the  thorax,  just  in  front  of  the  origin 
of  the  abdomen  (figs.  57,  58,  59,  h).  It  soon  be- 
comes hollow,  and  its  walls  exhibit  rhythmical  con- 
tractions. 

The  branchiae  are,  at  first,  simple  papillae  of  the  integu- 
ment of  the  region  from  which  they  take  their  rise. 
These  papillae  elongate  into  stems,  which  give  off  lateral 
filaments.  The  podobranchiaB  are  at  first  similar  to  the 
arthrobranchise,  but  an  outgrowth  soon  takes  place  near 
the  free  end  of  the  stem,  and  becomes  the  lamina,  while 
the  attached  end  enlarges  into  the  base. 

The  renal  organ  is  stated  to  arise  by  a  tubular  involu- 
tion of  the  epiblast,  which  soon  becomes  convoluted,  and 
gives  rise  to  the  green  gland. 

The  central  nervous  system  is  wholly  a  product  of  the 
epiblast.  The  cells  which  lie  at  the  sides  of  the  longi- 
tudinal groove  already  mentioned  (fig.  58,  mg),  grow  in- 
wards, and  give  rise  to  two  cords  which  are  at  first 
separate  from  one  another  and  continuous  with  the  rest 
of  the  epiblast.  At  the  front  end  of  the  groove  a 


DEVELOPMENT  OF   THE  NERVOUS  SYSTEM.  225 

depression  arises,  and  its  cells  form  a  mass  which  con- 
nects these  two  cords  in  front  of  the  mouth,  and  gives 
rise  to  the  cerebral  ganglia.  The  epiblastic  linings  of 
two  small  pits  (fig.  58,  o)  which  appear  very  early  on  the 
surface  of  the  procephalic  lobes,  are  also  carried  inwards 
Ln  the  same  way,  and,  uniting  with  the  foregoing, 
produce  the  optic  ganglia. 

The  cells  of  the  longitudinal  cords  become  differ- 
entiated into  nerve  fibres  and  nerve  cells,  and  the  latter, 
gathering  towards  certain  points,  give  rise  to  the  ganglia 
which  eventually  unite  in  the  middle  line.  By  degrees, 
the  ingrowth  of  epiblastic  cells,  from  which  all  these  struc- 
tures are  developed,  becomes  completely  separated  from 
the  rest  of  the  epiblast,  and  is  invested  by  mesoblastic 
cells.  The  central  nervous  system,  therefore,  in  a  crayfish, 
as  in  a  vertebrated  animal,  is  at  first,  as  a  part  of  the 
ectoderm,  morphologically  one  with  the  epidermis;  and  the 
deep  and  protected  position  which  it  occupies  in  the  adult 
is  only  a  consequence  of  the  mode  in  which  the  nervous 
portion  of  the  ectoderm  grows  inwards  and  becomes 
detached  from  the  epidermic  portion. 

The  visual  rods  of  the  eye  are  merely  modified  cells  of 
the  ectoderm.  The  auditory  sac  is  farmed  by  an  involu- 
tion of  the  ectoderm  of  the  basal  joint  of  the  antennule. 
At  birth  it  is  a  shallow  wide-mouthed  depression,  and 
contains  no  otoliths. 

Lastly,  the  reproductive  organs  result  from  the  segre- 
gation and  special  modification  of  cells  of  the  mesoblast 
16 


226  THE  MORPHOLOGY  OF  THE  CRAYFISH. 

behind  the  liver.  Rathke  states  that  the  sexual  apertures 
are  not  visible  until  the  young  crayfish  has  attained  the 
length  of  an  inch ;  and  that  the  first  pair  of  abdominal 
appendages  of  the  male  appear  still  later  in  the  form  of 
two  papillae,  which  gradually  elongate  and  take  on  their 
characteristic  forms. 


CHAPTEE  V. 

COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISU.- 
STRUCTURE  AND  THE  DEVELOPMENT  OF  THE  CRAY- 
FISH COMPARED  WITH  THOSE  OF  OTHER  LIVING 
BEINGS. 

UP  to  this  point,  our  attention  has  been  directed 
almost  exclusively  to  the  common  English  crayfish. 
Except  in  so  far  as  the  crayfish  is  dependent  for  its 
maintenance  upon  other  animals,  or  upon  plants,  we 
might  have  ignored  the  existence  of  all  living  things 
except  crayfishes.  But,  it  is  hardly  necessary  to  observe, 
that  innumerable  hosts  of  other  forms  of  life  not  only 
tenant  the  waters  and  the  dry  land,  but  throng  the  air ; 
and  that  all  the  cra}Tfishes  in  the  world  constitute  a  hardly 
appreciable  fraction  of  its  total  living  population. 

Common  observation  leads  us  to  see  that  these  multi- 
tudinous living  beings  differ  from  not-living  things  in 
many  ways ;  and  when  the  analysis  of  these  differences 
is  pushed  as  far  as  we  are  at  present  able  to  carry  it,  it 
shews  us  that  all  living  beings  agree  with  the  crayfish 
and  differ  from  not-living  things  in  the  same  particulars. 
Like  the  crayfish,  they  are  constantly  wasting  away  by 


228  THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

oxidation,  and  repairing  themselves  by  taking  into  their 
substance  the  matters  which  serve  them  for  food  ;  like 
the  crayfish,  they  shape  themselves  according  to  a  defi- 
nite pattern  of  external  form  and  internal  structure  ;  like 
the  crayfish,  they  give  off  germs  which  grow  and  develope 
into  the  shapes  characteristic  of  the  adult.  No  mi- 
neral matter  is  maintained  in  this  fashion ;  nor  grows  in 
the  same  way ;  nor  undergoes  this  kind  of  development ; 
nor  multiplies  its  kind  by  any  such  process  of  reproduc- 
tion. 

Again,  common  observation  early  leads  to  the  discri- 
mination of  living  things  into  two  great  divisions.  No- 
body confounds  ordinary  animals  with  ordinary  plants, 
nor  doubts  that  the  crayfish  belongs  to  the  former  cate- 
gory and  the  waterweed  to  the  latter.  If  a  living  thing 
moves  and  possesses  a  digestive  receptacle,  it  is  held  to 
be  an  animal ;  if  it  is  motionless  and  draws  its  nourish- 
ment directly  from  the  substances  which  are  in  contact 
with  its  outer  surface,  it  is  held  to  be  a  plant.  We  need 
not  inquire,  at  present,  how  far  this  rough  definition  of 
the  differences  which  separate  animals  from  plants  holds 
good.  Accepting  it  for  the  moment,  it  is  obvious  that 
the  crayfish  is  unquestionably  an  animal, — as  much  an 
animal  as  the  vole,  the  perch,  and  the  pond-snail,  which 
inhabit  the  same  waters.  Moreover,  the  crayfish  has,  in 
common  with  these  animals,  not  merely  the  motor  and 
digestive  powers  characteristic  of  animality,  but  they  all, 
like  it,  possess  a  complete  alimentary  canal;  special  appa- 


COMPARATIVE  MORPHOLOGY.  229 

ratus  for  the  circulation  and  the  aeration  of  the  hlood ; 
a  nervous  system  with  sense-organs  ;  muscles  and  motor 
mechanisms ;  reproductive  organs.  Regarded  as  pieces 
of  physiological  apparatus,  there  is  a  striking  similarity 
between  all  three.  But,  as  has  already  been  hinted  in 
the  preceding  chapter,  if  we  look  at  them  from  a  purely 
morphological  point  of  view,  the  differences  between  the 
crayfish,  the  perch,  and  the  pond-snail,  appear  at  first 
sight  so  great,  that  it  may  be  difficult  to  imagine  that  the 
plan  of  structure  of  the  first  can  have  any  relation  to 
that  of  either  of  the  last  two.  On  the  other  hand,  if  the 
crayfish  is  compared  with  the  water-beetle,  notwithstand- 
ing wide  differences,  many  points  of  similarity  between 
the  two  will  manifest  themselves;  while,  if  a  small 
lobster  is  set  side  by  side  with  a  crayfish,  an  unpractised 
observer,  though  he  will  readily  see  that  the  two  animals 
are  somewhat  different,  may  be  a  long  time  in  making 
out  the  exact  nature  of  the  differences. 

Thus  there  are  degrees  of  likeness  and  unlikeness 
among  animals,  in  respect  of  their  outward  form  and 
internal  structure,  or,  in  other  words,  in  their  morpho- 
logy. The  lobster  is  very  like  a  crayfish,  the  beetle  is 
remotely  like  one;  the  pond-snail  and  the  perch  are 
extremely  unlike  crayfishes.  Facts  of  this  kind  are  com- 
monly expressed  in  the  language  of  zoologists,  by  saying 
that  the  lobster  and  the  crayfish  are  closely  allied 
forms ;  that  the  beetle  and  the  crayfish  present  a  re- 
mote affinity ;  and  that  there  is  no  affinity  between  the 


2.30  THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

crayfish  and   the   pond-snail,    or   the   crayfish   and   the 
perch. 

The  exact  determination  of  the  resemhlances  and 
differences  of  animal  forms  by  the  comparison  of  the 
structure  and  the  development  of  one  with  those  of 
another,  is  the  business  of  comparative  morphology. 
Morphological  comparison,  fully  and  thoroughly  worked 
out,  furnishes  us  with  the  means  of  estimating  the 
position  of  any  one  animal  in  relation  to  all  the 
rest;  while  it  shews  us  with  what  forms  that  animal 
is  nearly,  and  with  what  it  is  remotely,  allied :  ap- 
plied to  all  animals,  it  furnishes  us  with  a  kind  of 
map,  upon  which  animals  are  arranged  in  the  order  of 
their  respective  affinities ;  or  a  classification,  in  which 
they  are  grouped  in  that  order.  For  the  purpose  of 
developing  the  results  of  comparative  morphology  in  the 
case  of  the  crayfish,  it  will  be  convenient  to  bring  toge* 
ther,  in  a  summary  form,  those  points  of  form  and  struc- 
ture, many  of  which  have  already  been  referred  to  and 
which  characterise  it  as  a  separate  kind  of  animal. 

Full-grown  English  crayfishes  usually  measure  about 
three  inches  and  a  half  from  the  extremity  of  the  rostrum 
in  front  to  that  of  the  telson  behind.  The  largest 
specimen  I  have  met  with  measured  four  inches.*  The 

*  The  dimensions  of  crayfishes  at  successive  ages  given  at  p.  31, 
beginning  at  the  words  "  By  the  end  of  the  year,"  refer  to  the  "  <$cre- 
visse  a  pieds  rouges  "  of  France  ;  not  to  the  English  crayfish,  which  ia 


DISTINCTIVE   CHARACTERS  OF  THE  CR 1YFISH.       231 

males  are  commonly  somewhat  larger,  and  they  almost 
always  have  longer  and  stronger  forceps  than  the 
females.  The  general  colour  of  the  integument  varies 
from  a  light  reddish-brown  to  a  dark  olive-green ;  and 
the  hue  of  the  tergal  surface  of  the  body  and  limbs  is 
always  deeper  than  that  of  the  sternal  surface,  which  is 
often  light  yellowish-green,  with  more  or  less  red  at  the 
extremities  of  the  forceps.  The  greenish  hue  of  the 
sternal  surface  occasionally  passes  into  yellow  in  the 
thorax  and  into  blue  in  the  abdomen. 

The  distance  from  the  orbit  to  the  posterior  margin  of 
the  carapace  is  nearly  equal  to  that  from  the  posterior 
margin  of  the  carapace  to  the  base  of  the  telson,  when 
the  abdomen  is  fully  extended,  but  this  measurement  of 
the  carapace  is  commonly  greater  than  that  of  the  abdo- 
men in  the  males  and  less  in  the  females. 

The  general  contour  of  the  carapace  (fig.  61),  without 
the  rostrum,  is  that  of  an  oval,  truncated  at  the  ends : 
the  anterior  end  being  narrower  than  the  posterior.  Its 
surface  is  evenly  arched  from  side  to  side.  The  greatest 
breadth  of  the  carapace  lies  midway  between  the  cervical 
groove  and  its  posterior  edge.  Its  greatest  vertical  depth 
is  on  a  level  with  the  transverse  portion  of  the  cervical 
groove. 

The  length  of  the  rostrum,  measured  from  the  orbit 

considerably  smaller.  Doubtless,  the  proportional  rate  of  increment  ia 
much  the  same,  in  the  two  kinds  ;  but  in  the  Foiglish  crayfish  it  haa 
not  been  actually  ascertained. 


232  THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

to  its  extremity,  is  greater  than  half  the  distance  from 
the  orbit  to  the  cervical  groove.  It  is  trihedral  in  sec- 
tion, and  its  free  end  is  slightly  curved  upwards  (fig.  41). 
It  gradually  becomes  narrower  for  about  three-fourths  of 
its  whole  length.  At  this  point  it  has  rather  less  than 
half  the  width  which  it  has  at  its  base  (fig.  61,  A)  ;  and  its 
raised,  granular  and  sometimes  distinctly  serrated  margins 
are  produced  into  two  obliquely  directed  spines,  one  on 
each  side.  Beyond  these,  the  rostrum  rapidly  narrows 
to  a  fine  point ;  and  this  part  of  the  rostrum  is  equal  in 
length  to  the  width  between  the  two  spines. 

The  tergal  surface  of  the  rostrum  is  flattened  and 
slightly  excavated  from  side  to  side,  except  in  its  an- 
terior half,  where  it  presents  a  granular  or  finely  ser- 
rated median  ridge,  which  gradually  passes  into  a  low 
elevation  in  the  posterior  half,  and,  as  such,  may  gener- 
ally be  traced  on  to  the  cephalic  region  of  the  carapace. 
The  inclined  sides  of  the  rostrum  meet  ventrally  in  a 
sharp  edge,  convex  from  before  backwards  ;  the  posterior 
half  of  this  edge  gives  rise  to  a  small,  usually  bifurcated, 
spine,  which  descends  between  the  eye-stalks  (fig.  41). 
The  raised  and  granulated  lateral  margins  of  the  rostrum 
are  continued  back  on  to  the  carapace  for  a  short  distance, 
as  two  linear  ridges  (fig.  61,  A).  Parallel  with  each  of 
these  ridges,  and  close  to  it,  there  is  another  longitudinal 
elevation  (a,  b),  the  anterior  end  of  which  is  raised  into  a 
prominent  spine  (a),  which  is  situated  immediately  behind 
the  orbit,  and  may,  therefore,  be  termed  the  post-orbital 


DISTINCTIVE   CHARACTERS   OF  THE  CRAYFISH.        233 


The  elevation  itself  may  be  distinguished  as  the 
post-orbital  ridge.     The  flattened  surface  of  this  ridge  is 
marked  by  a  longitudinal   depression   or  groove.     The 
ABC 


FlG.  61.— A.  D,  &  Gr,A*tacvs  torrentium  ;  B,  E,  &  H,  A.  noUlis;  C,  F,  & 
I,  .4.  nirjrwrnx  (nat.  size).  A — C,  Dorsal  views  of  carapace  ;  D — F, 
side  views  of  third  abdominal  somites  ;  G — I,  Dorsal  views  of  telson. 
a,  b,  post-orbital  ridge  and  spines  ;  c,  branchio- cardiac  grooves 
inclosing  the  areola. 


234  THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

posterior  end  of  the  ridge  passes  into  a  somewhat  broader 
and  less  marked  elevation,  the  hinder  end  of  which  turns 
inwards,  and  then  comes  to  an  end  at  a  point  midway 
between  the  orbit  and  the  cervical  groove.  Generally 
this  hinder  elevation  appears  like  a  mere  continuation 
of  the  post-orbital  ridge ;  but,  sometimes,  the  two  are 
separated  by  a  distinct  depression.  I  have  never  seen 
any  prominent  spine  upon  the  posterior  elevation,  though 
it  is  sometimes  minutely  spinulose.  The  post-orbital 
ridges  of  each  side,  viewed  together,  give  rise  to  a  cha- 
racteristic lyrate  mark  upon  the  cephalic  region  of  the 
carapace. 

A  faintly  marked,  curved,  linear  depression  runs  from 
the  hinder  end  of  the  post-orbital  ridge,  at  first  directly 
downwards,  and  then  curves  backwards  to  the  cervical 
groove.  It  corresponds  with  the  anterior  and  inferior 
boundary  of  the  attachment  of  the  adductor  muscle  of 
the  mandible. 

Below  the  level  of  this,  and  immediately  behind  the 
cervical  groove,  there  are  usually  three  spines,  arranged 
in  a  series,  which  follow  the  cervical  groove.  The  points 
of  all  are  directed  obliquely  forwards,  and  the  lowest  is 
the  largest.  Sometimes  there  is  only  one  prominent 
spine,  with  one  or  two  very  small  ones ;  sometimes  there 
are  as  many  as  five  of  these  cervical  spines. 

The  cardiac  region  is  marked  out  by  two  grooves  which 
run  backwards  from  the  cervical  groove  (fig.  61,  A,  c),  and 
terminate  at  a  considerable  distance  from  the  posterioi 


DISTINCTIVE  CHARACTERS   OF  THE  CRAYFISH.        235 

edge  of  the  carapace.  Each  groove  runs,  at  first,  obliquely 
inwards,  and  then  takes  a  straight  course  parallel  with  its 
fellow.  The  area  thus  defined  is  termed  the  areola  ;  its 
breadth  is  equal  to  about  one-third  of  the  total  transverse 
diameter  of  the  carapace  in  this  region. 

No  such  distinct  lines  indicate  the  lateral  boundary  of 
the  region  in  front  of  the  cervical  groove  which  answers 
to  the  stomach.  But  the  middle  part  of  the  carapace, 
or  that  which  is  comprised  in  the  gastric  and  cardiac 
regions,  has  its  surface  sculptured  in  a  different  way 
from  the  branchiostegites  and  the  lateral  regions  of  the 
head.  In  the  former,  the  surface  is  excavated  by  shal- 
low pits,  separated  by  relatively  broad  flat-topped  ridges  ; 
but,  in  the  latter,  the  ridges  become  more  prominent, 
and  take  the  form  of  tubercles,  the  apices  of  which  are 
directed  forwards.  Minute  setae  spring  from  the  depres- 
sions between  these  tubercles. 

The  branchiostegite  has  a  thickened  rim,  which  is 
strongest  below  and  behind  (fig.  1).  The  free  edge  of 
this  rim  is  fringed  with  close-set  setae. 

The  pleura  of  the  second  to  the  sixth  abdominal 
somites  are  broadly  lanceolate  and  obtusely  pointed  at 
their  free  ends  (fig.  61,  D) ;  the  anterior  edge  is  longer 
and  more  convex  than  the  posterior  edge.  In  the  females, 
the  pleura  are  larger,  and  are  directed  more  outwards  and 
less  downwards  than  in  the  males.  The  pleura  of  the 
second  somite  are  much  larger  than  the  rest,  and  over- 
lap the  very  small  pleura  of  the  first  somite  (fig.  1).  The 


236  THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

pleura  of  the  sixth  somite  are  narrow,  and  their  posterior 
edges  are  concave. 

The  pits  and  setae  of  the  cuticle  which  clothes  the 
tergal  surfaces  of  the  abdominal  somites  are  so  few  and 
scattered,  that  the  latter  appear  almost  smooth,  In  the 
telson,  however,  especially  in  its  posterior  division,  the 
markings  are  coarser  and  the  setae  more  apparent. 

The  telson  (fig.  61,  G)  presents  an  anterior  quadrate  divi- 
sion and  a  posterior  half-oval  part,  the  free  curved  edge 
of  which  is  beset  with  long  setae,  and  is  sometimes  slightly 
notched  in  the  middle.  The  posterior  division  is  freely 
movable  upon  the  anterior,  in  consequence  of  the  thin- 
ness and  pliability  of  the  cuticle  along  a  transverse  line 
which  joins  tne  postero-external  angles  of  the  anterior 
division,  each  of  which  is  produced  into  two  strong  spines, 
of  which  the  outer  is  the  longer.  The  length  of  the  pos- 
terior division  of  the  telson,  measured  from  the  middle 
of  the  suture,  is  equal  to,  or  but  very  little  less  than, 
that  of  the  anterior  division. 

On  the  under  side  of  the  head,  the  basal  joints  of  the 
antennules  are  visible,  internal  to  those  of  the  antennae, 
but  the  attachment  of  the  latter  is  behind  and  below 
that  of  the  former  (fig.  3,  A).  Behind  these,  and  in 
front  of  the  mouth,  the  epistoma  (fig.  39,  A,  II,  III) 
presents  a  broad  area  of  a  pentagonal  form.  The  pos- 
terior boundary  of  this  area  is  formed  by  two  thickened 
transverse  ridges,  which  meet  on  the  middle  line  at  a 
very  open  angle,  the  upex  of  which  is  turned  forwards. 


DISTINCTIVE  CHARACTERS  OF  THE  CRAYFISH.      237 

The  posterior  edges  of  these  ridges  are  continuous  with 
the  lahrum.  The  anterior  margin  is  produced  in  the 
middle  into  a  fleur  de  lys  shaped  process,  the  summit 
of  which  terminates  between  the  antennules.  At  the 
sides  of  this  process,  the  anterior  margin  of  the  epis- 
toma  is  deeply  excavated  to  receive  the  basal  joints  of 
the  antennas.  Following  the  contours  of  these  excavated 
margins,  the  surface  of  the  epistoma  presents  two  lateral 
convexities.  The  widest  and  most  prominent  part  of  each 
of  these  lies  towards  the  outer  edge  of  the  epistoma, 
and  is  produced  into  a  conical  spine.  Sometimes 
there  is  a  second  smaller  spine  beside  the  principal  one. 
Between  the  two  convexities  lies  a  triangular  median 
depressed  area. 

The  distance  from  the  apex  of  the  anterior  median 
process  to  the  posterior  ridge  is  equal  to  a  little  more 
than  half  the  width  of  the  epistoma. 

The  corneal  surface  of  the  eye  is  transversely  elongated 
°nd  reniforin,  and  its  pigment  is  black.  The  eye-stalks 
are  much  broader  at  their  bases  than  at  their  comeal 
ends  (fig.  48,  A).  The  antennules  are  about  twice  as  long 
as  the  rostrum.  The  tergal  surface  of  the  trihedral 
basal  joint  of  the  antennule,  on  which  the  eye-stalk  rests, 
is  concave;  the  outer  surface  is  convex,  the  inner  flat 
(figs.  26,  A,  and  48,  B).  Near  the  anterior  end  of  the 
sternal  edge  which  separates  the  two  latter  faces,  there 
is  a  strong  curved  spine  directed  forwards  (fig.  48,  B,  a). 
When  the  setae,  which  proceed  from  the  outer  edge  of 


238     THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

the  auditory  aperture  and  hide  it,  are  removed,  it  is 
seen  to  be  a  wide,  somewhat  triangular  qleft,  which  occu- 
pies the  greater  part  of  the  hinder  half  of  the  tergal 
surface  of  the  basal  joint  (fig.  26,  A). 

The  exopodites,  or  squames,  of  the  antennae  extend  aj 
far  as  the  apex  of  the  rostrum,  or  even  project  beyond  it, 
when  they  are  turned  forwards,  while  they  reach  to  the 
commencement  of  the  filament  of  the  endopodite  (Frontis- 
piece). The  squame  is  fully  twice  as  long  as  it  is  broad, 
with  a  general  convexity  of  its  tergal  and  concavity  of  its 
sternal  surface.  The  outer  edge  is  straight  and  thick,  the 
inner,  which  is  fringed  with  long  setae,  is  convex  and  thin 
(fig.  48,  C).  Where  these  two  edges  join  in  front,  the 
squame  is  produced  into  a  strong  spine.  A  thick  outer 
portion  of  the  squame  is  marked  off  from  the  thinner 
inner  portion  by  a  longitudinal  groove  on  the  tergal  side, 
and  by  a  strong  ridge  on  the  sternal  side.  One  or  two 
small  spines  generally  project  from  the  posterior  and 
external  angle  of  the  squame ;  but  they  may  be  very 
small  or  absent  in  individual  specimens.  Close  beneath 
these,  the  outer  angle  of  the  next  joint  is  produced  intc 
a  strong  spine.  When  the  abdomen  is  straightened  out, 
if  the  antennae  are  turned  back  as  far  as  they  will  go 
without  damage,  the  ends  of  their  filaments  usually  reach 
the  tergum  of  the  third  somite  of  the  abdomen  (Frontis- 
piece). I  have  not  observed  any  difference  between  the 
sexes  in  this  respect. 

The  inner  edge  of  the  ischiopodite  of  the  third  maxilli- 


DISTINCTIVE  CHARACTERS   OF  THE  CRAYFISH.        239 

pede  is  strongly  serrated  and  wider  in  front  than  behind 
(tig.  44) ;  the  meropodite  possesses  four  or  five  spines 
in  the  same  region ;  and  there  are  one  or  two  spines  at 
the  distal  end  of  the  carpopodite.  When  straightened 
out,  the  maxillipedes  extend  as  far  as,  or  even  beyond, 
the  end  of  the  rostrum. 

The  inner  or  sternal  edge  of  the  ischiopodite  of  the 
forceps  is  serrated  ;  that  of  the  meropodite  presents  two 
rows  of  spines,  the  inner  small  and  numerous,  the  outer 
large  and  few.  There  are  several  strong  spines  at  the 
anterior  end  of  the  outer  or  tergal  face  of  this  joint.  The 
carpopodite  has  two  strong  spines  on  its  under  or  sternal 
surface,  while  its  sharp  inner  edge  presents  many  strong 
spines.  Its  upper  surface  is  marked  by  a  longitudinal  de- 
pression, and  is  beset  with  sharp  tubercles.  The  length 
of  the  propodite,  from  its  base  to  the  extremity  of 
the  Lxed  claw  of  the  chela,  measures  rather  more  than 
twice  as  much  as  the  extreme  breadth  of  its  base,  the 
thickness  of  which  is  less  than  a  third  of  this  length 
(fig.  20,  p.  93).  The  external  angular  process,  or  fixed 
claw,  is  of  the  same  length  as  the  base,  or  a  little  shorter. 
Its  inner  edge  is  sharp  and  spinose,  and  the  outer  more 
rounded  and  simply  tuberculated.  The  apex  of  the  fixed 
claw  is  produced  into  a  slightly  incurved  spine.  Its 
inner  edge  has  a  sinuous  curvature,  convex  posteriorly, 
concave  anteriorly,  and  bears  a  series  of  rounded  tubercles, 
of  which  one  near  the  summit  of  the  convexity,  and  one 
near  the  apex  of  the  claw,  are  the  most  prominent. 


240     THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

The  apex  of  the  dactylopodite,  like  that  of  the  propc- 
dite,  is  formed  by  a  slightly  incurved  spine  (fig.  20),  while 
its  outer,  sharper,  edge  presents  a  curvature,  the  inverse 
of  that  of  the  edge  of  the  fixed  claw  against  which  it  is 
applied.  This  edge  is  beset  with  rounded  tubercles,  the 
most  prominent  of  which  are  one  at  the  beginning,  and 
one  at  the  end  of  the  concave  posterior  moiety  of  the  edge. 
When  the  dactylopodite  is  brought  up  to  the  fixed  claw, 
these  tubercles  lie,  one  in  front  of  and  one  behind  the 
chief  tubercle  of  the  convexity  of  the  latter.  The  whole 
surface  of  the  propodite  and  dactylopodite  is  covered 
with  minute  elevations,  those  of  the  upper  surface  being 
much  more  prominent  than  those  of  the  lower  surface. 

The  length  of  the  fully  extended  forceps  generally 
equals  the  distance  between  the  posterior  margin  of  the 
orbit  and  the  base  of  the  telson,  in  well  characterized 
males  ;  and,  in  individual  examples,  they  are  even  longer  ; 
while  it  may  not  be  greater  than  the  distance  between 
the  orbit  and  the  hinder  edge  of  the  fourth  abdominal 
somite,  in  females ;  and,  in  massiveness  arid  strength,  the 
difference  of  the  forceps  in  the  two  sexes  is  still  more 
remarkable  (fig.  2).  Moreover  there  is  a  good  deal  of 
variation  in  the  form  and  size  of  the  chela?  in  individual 
males.  The  right  and  left  chelse  present  no  important 
differences. 

The  ischiopodites  of  the  four  succeeding  thoracic  limbs 
are  devoid  of  any  recurved  spines  in  either  sex  (Front., 
fig.  46),  The  first  pair  are  the  stoutest,  the  second  the 


DISTINCTIVE  CHAEACTERS  OF  THE  CRAYFISH.  241 

longest :  and  when  the  latter  are  spread  out  at  right 
angles  to  the  body,  the  distance  from  tip  to  tip  of  the  dac- 
tylopodites  is  equal  to,  or  rather  greater  than,  the  extreme 
length  of  the  body  from  the  apex  of  the  rostrum  to  the 
posterior  edge  of  the  telson,  in  both  sexes.  In  both  sexes, 
the  length  of  the  swimmerets  hardly  exceeds  half  the  trans- 
verse diameter  of  the  somites  to  which  they  are  attached. 

The  exopodites  of  the  appendages  of  the  sixth  abdo- 
minal somite  (the  extreme  length  of  which  is  rather 
greater  than  that  of  the  telson)  are  divided  into  a  larger 
proximal,  and  a  smaller  distal  portion  (fig.  37,  F,  p.  144). 
The  latter  is  about  half  as  long  as  the  former,  and  has  a 
rounded  free  edge,  setose  like  that  of  the  telson.  There 
is  a  complete  flexible  hinge  between  the  two  portions, 
and  the  overlapping  free  edge  of  the  proximal  portion, 
which  is  slightly  concave,  is  beset  with  conical  spines, 
the  outermost  of  which  are  the  longest.  The  endopodite 
has  a  spine  at  the  junction  of  its  outer  straight  edge 
with  the  terminal  setose  convex  edge.  A  faintly  marked 
longitudinal  median  ridge,  or  keel,  ends  close  to  the 
margin  in  a  minute  spine.  The  tergal  distal  edge  of 
the  protopodite  is  deeply  bilobed,  and  the  inner  lobe 
ends  in  two  spines,  while  the  outer,  shorter  and  broader 
lobe,  is  minutely  serrated. 

In  addition  to  the  characters  distinctive  of  sex,  which 

have  already  been  fully  detailed  (pp.  7,  20,  and  145),  there 

is  a  marked  difference  in  the  form  of  the  sterna  of  the  three 

posterior  thoracic  somites  between  the  males  and  females. 

17 


242     THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYF  LS11. 

Comparing  a  male  and  a  female  of  the  same  size,  the 
triangular  area  between  the  bases  of  the  penultimate  and 
ante-penultimate  thoracic  limbs  is  considerably  broader 
at  the  base  in  the  female.  In  both  sexes,  the  hinder 
part  of  the  penultimate  sternum  is  a  rounded  transverse 
ridge  separated  by  a  groove  from  the  anterior  part ;  but 
this  ridge  is  much  larger  and  more  prominent  in  the 
female  than  in  the  male,  and  it  is  often  obscurely  divided 
into  two  lobes  by  a  median  depression.  Moreover,  there 
are  but  few  setae  on  this  region  in  the  female ;  while,  in 
the  male,  the  setse  are  long  and  numerous. 

The  sternum  of  the  last  thoracic  somite  of  the  female 
is  divided  by  a  transverse  groove  into  two  parts,  of  which 
the  posterior,  viewed  from  the  sternal  aspect,  has  the 
form  of  a  transverse  elongated  ridge,  which  narrows  to 
each  end,  is  moderately  convex  in  the  middle,  and  is 
almost  free  from  setae.  In  the  male,  the  corresponding 
posterior  division  of  the  last  thoracic  sternum  is  produced 
downwards  and  forwards  into  a  rounded  eminence  which 
gives  attachment  to  a  sort  of  brush  of  long  setse  (fig.  35, 
p.  136). 

The  importance  of  this  long  enumeration  of  minute 
details*  will  appear  by  and  by.  It  is  simply  a  statement  of 
the  more  obvious  external  characters  in  which  all  the 
full-grown  English  crayfishes  which  have  come  under  my 

*  The  student  of  systematic  zoology  will  find  the  comparison  of  a 
lobster  with  a  crayfish  in  all  the  points  mentioned  to  be  an  excellent 
training  of  the  faculty  of  observation. 


THE   GENERAL  NAME,   SPECIES.  243 

notice  agree.  No  one  of  these  individual  crayfishes  was 
exactly  like  the  other;  and  to  give  an  account  of  any 
single  crayfish  as  it  existed  in  nature,  its  special  peculiari- 
ties must  he  added  to  the  list  of  characters  given  ahove ; 
which,  considered  together  with  the  facts  of  structure 
discussed  in  previous  chapters,  constitutes  a  definition, 
or  diagnosis,  of  the  English  kind,  or  species,  of  crayfish. 
It  follows  that  the  species,  regarded  as  the  sum  of  the 
morphological  characters  in  question  and  nothing  else, 
does  not  exist  in  nature  ;  hut  that  it  is  an  abstraction, 
obtained  by  separating  the  structural  characters  in  which 
the  actual  existences — the  individual  crayfishes — agree, 
from  those  in  which  they  differ,  and  neglecting  the  latter. 
A  diagram,  embodying  the  totality  of  the  structural 
characters  thus  determined  by  observation  to  be  common 
to  all  our  crayfishes,  might  be  constructed ;  and  it 
would  be  a  picture  of  nothing  which  ever  existed  in 
nature;  though-  it  would  serve  as  a  very  complete 
plan  of  the  structure  of  all  the  crayfishes  which  are  to 
be  found  in  this  country.  The  morphological  definition 
of  a  species  is,  in  fact,  nothing  but  a  description  of  the 
plan  of  structure  which  characterises  all  the  individuals 
of  that  species. 

( California  is  separated  from  these  islands  bya  third  of  the 
jii  cumference  of  the  globe,  one-half  of  the  interval  being 
u  ecu  pied  by  the  broad  North  Atlantic  ocean.  The  fresh 
waters  of  California,  however,  contain  crayfishes  which  are 


244     THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRA.YFISH. 

so  like  our  own,  that  it  is  necessary  to  compare  the  two  in 
every  point  mentioned  in  the  foregoing  description  in 
order  to  estimate  the  value  of  the  differences  which  they 
present.  Thus,  to  take  one  of  the  kinds  of  crayfishes  found 
in  California,  which  has  been  called  Astacus  nigrescens ; 
the  general  structure  of  the  animal  may  be  described  in 
precisely  the  same  terms  as  those  used  for  the  English 
crayfish.  Even  the  branchiae  present  no  important 
difference,  except  that  the  rudimentary  pleurobranchise 
are  rather  more  conspicuous ;  and  that  there  is  a  third 
small  one,  in  front  of  the  two  which  correspond  with  those 
possessed  by  the  English  crayfish. 

The  Californian  crayfish  is  larger  and  somewhat  diffe- 
rently coloured,  the  undersides  of  the  forceps  particularly 
presenting  a  red  hue.  The  limbs,  and  especially  the 
forceps  of  the  males,  are  relatively  longer ;  the  chelae  of 
the  forceps  have  more  slender  proportions ;  the  areola  is 
narrower  relatively  to  the  transverse  diameter  of  the 
carapace  (fig.  61,  C).  More  definite  distinctions  are  to  be 
found  in  the  rostrum,  which  is  almost  parallel- sided  for 
two-thirds  of  its  length,  then  gives  off  two  strong  lateral 
spines  and  suddenly  narrows  to  its  apex.  Behind  these 
spines,  the  raised  lateral  edges  of  the  rostrum  present  five 
or  six  other  spines  which  diminish  in  size  from  before 
backwards.  The  postorbital  spine  is  very  prominent, 
but  the  ridge  is  represented,  in  front,  by  the  base  of  this 
spine,  which  is  slightly  grooved ;  and  behind,  by  a  distinct 
spine  which  is  not  so  strong  as  the  postorbital  spine. 


ASTACUS   NIGRESCENS. 


245 


There  are  no  cervical  spines,  and  the  middle  part  of 
the  cervical  groove  is  angulated  backwards  instead  of 
being  transverse. 


FIG.  62.  A  &  D,  Astacns  torrent ium •;  B  &  E,  A.  ndbilis ;  C  &  F, 
A.  nigrescens.  A — C,  1st  abdominal  appendage  of  the  male  ;  D — F, 
endopodite  of  second  appendage  (  x  3).  a,  anterior,  and  £>,  posterior  rolled 
edge :  c,  d,  e,  corresponding-  parts  of  the  appendages  in  each  species  ; 
/,  rolled  plate  of  endopodite  ;  g,  terminal  division  of  endopodite. 

The  abdominal  pleura  are  narrow,  equal-sided,  and 
acutely  pointed  in  the  males  (fig.  61,  F) — slightly 
broader,  more  obtuse,  and  with  the  anterior  edges 


246  THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

rather  more  convex  than  the  posterior,  in  the  females. 
The  tergal  surface  of  the  telson  is  not  divided  into  two 
parts  by  a  suture  (fig.  61,  I).  The  anterior  process  of 
the  epistoma  is  of  a  broad  rhomboidal  shape,  and  there 
are  no  distinct  lateral  spines. 

The  squame  of  the  antenna  is  not  so  broad  relatively 
to  its  length ;  its  inner  edge  is  less  convex,  and  its  outer 
edge  is  slightly  concave ;  the  outer  basal  angle  is  sharp 
but  not  produced  into  a  spine.  The  opposed  edges  of 
the  fixed  and  movable  claws  of  the  chelae  of  the  forceps 
are  almost  straight  and  present  no  conspicuous  tubercles. 
In  the  males,  the  forceps  are  vastly  larger  than  in  the 
females,  and  the  two  claws  of  the  chelse  are  bowed  out,  so 
that  a  wide  interval  is  left  when  their  apices  are  applied 
together ;  in  the  females,  the  claws  are  straight  and  the 
edges  fit  together,  leaving  no  interval.  Both  the  upper  and 
the  under  surfaces  of  the  claws  are  almost  smooth.  The 
median  ridge  of  the  endopodite  of  the  sixth  abdominal 
appendage  is  more  marked,  and  ends  close  to  the  margin 
in  a  small  prominent  spine. 

In  the  females,  the  posterior  division  of  the  sternum  of 
the  penultimate  thoracic  somite  is  prominent  and  deeply 
bilobed ;  and  there  are  some  small  differences  in  form  in 
the  abdominal  appendages  of  the  males.  More  especially, 
the  rolled  inner  process  of  the  endopodite  of  the  second 
appendage  (fig.  62  F,  /)  is  disposed  very  obliquely,  and 
its  open  mouth  is  on  a  level  with  the  base  of  the  jointed 
part  of  the  endopodite  (g)  instead  of  reaching  almost  to 


THE   GENERAL    NAME,   GENUS.  247 

the  free  end  of  the  latter  and  being  nearly  parallel  with  it. 
In  the  first  appendage  (C),  the  anterior  rolled  edge  (a) 
more  closely  embraces  the  posterior  (£>),  and  the  groove 
is  more  completely  converted  into  a  tube. 

It  will  be  observed  that  the  differences  between  the 
English  and  the  Californian  crayfishes  amount  to  ex- 
ceedingl}r  little  ;  but,  on  the  assumption  that  these  differ- 
ences are  constant,  and  that  no  transitional  forms  between 
the  English  and  the  Californian  crayfishes  are  to  be 
met  with,  the  individuals  which  present  the  characteristic 
peculiarities  of  the  latter  are  said  to  form  a  distinct  species, 
Astacus  nigresctas ;  and  the  definition  of  that  species  is, 
like  that  of  the  English  species,  a  morphological  abstrac- 
tion, embodying  an  account  of  the  plan  of  that  species, 
so  far  as  it  is  distinct  from  that  of  other  crayfishes. 

We  shall  see  by  and  by  that  there  are  sundry  other 
kinds  of  crayfishes,  which  differ  no  more  from  the  English 
or  the  Californian  kinds,  than  these  do  from  one  an- 
other ;  and,  therefore,  they  are  all  grouped  as  species  of 
the  one  genus,  Astacus. 

If,  leaving  California,  we  cross  the  Rocky  Mountains 
and  enter  the  eastern  States  of  the  North  American 
Union,  many  sorts  of  crayfishes,  which  would  at  once  be 
recognised  as  such  by  any  English  visitor,  will  be  found 
to  be  abundant.  But  on  careful  examination  it  will  be 
discovered  that  ah1  of  these  differ,  both  from  the  English 
crayfish,  and  from  Astacus  nigrescens,  to  a  much  greater 


2  1-8  THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

extent  than  those  do  from  one .  another.     The  gills  are. 
i'i  fact,  reduced  to  seventeen  on  each  side,  in  consequence 


FIG.  63.     Cambants  Clarkil,  male  (^  nat.  size),  after  Hagen. 
of  the  absence  of  the  pleuro-branchia  of  the  last  thoracic 
somite;    and  there  are  some  other  differences  to  which 
it  is  not  needful  to  refer  at  present.      It  is  convenient  to 


THE  ABSTRACTIONS,   SPECIES   AND  GENUS.  249 

distinguish  these  seventeen-gilled  cra}"fishes,  as  a  whole, 
from  the  eighteen-gilled  species ;  and  this  is  effected  by 
changing  the  generic  name.  They  are  no  longer  called 
Astacus,  but  Cambarus  (fig.  63). 

All  the  individual  crayfish  referred  to  thus  far,  there- 
fore, have  been  sorted  out,  first  into  the  groups  termed 
species  ;  and  then  these  species  have  been  further  sorted 
into  two  divisions,  termed  genera.  Each  genus  is  an 
abstraction,  formed  by  summing  up  the  common  char- 
acters of  the  species  which  it  includes,  just  as  each 
species  is  an  abstraction,  composed  of  the  common 
characters  of  the  individuals  which  belong  to  it ;  and 
the  one  has  no  more  existence  in  nature  than  the  other. 
The  definition  of  the  genus  is  simply  a  statement  of 
the  plan  of  structure  which  is  common  to  all  the  species 
included  under  that  genus ;  just  as  the  definition  of  the 
species  is  a  statement  of  the  common  plan  of  structure 
which  runs  throughout  the  individuals  which  compose 
the  species. 

Again,  crayfishes  are  found  in  the  fresh  waters  of  the 
Southern  hemisphere ;  and  almost  the  whole  of  what 
has  been  said  respecting  the  structure  of  the  English  cray- 
fish applies  to  these  ;  in  other  words,  their  general  plan  is 
the  same.  But,  in  these  southern  crayfishes,  the  podo- 
branchiae  have  no  distinct  lamina,  and  the  first  somite  of 
the  abdomen  is  devoid  of  appendages  in  both  sexes.  The 
southern  crayfishes,  like  those  of  the  Northern  hemi- 
sphere, are  divisible  into  many  species ;  and  these  species 


250  THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

are  susceptible  of  being  grouped  into   six  genera — Asia- 
coides  (fig.  65),  Astacopsis,  Champs,  Pamstacus  (fig.  64), 


FIG.  <M.—Parastacus  Irasiliensis  (\  nat.  size).     From  southern  Brazil. 

Engceus,  and  Paranephrops — on  the  same  principle  as 
that  which  has  led  to  the  grouping  of  the  Northern  forms 
into  two  genera.  But  ths  same  convenience  which  has 


FIG.  65. — Astacoides  madagascartiHsis  (f  nat.  size).     From 


252  THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

led  to  the  association  of  groups  of  similar  species  into 
genera,  has  given  rise  to  the  combination  of  allied  genera 
into  higher  groups,  which  are  termed  Families.  It  is 
ohvious  that  the  definition  of  a  family,  as  a  statement  of 
the  characters  in  which  a  certain  number  of  genera  agree, 
is  another  morphological  abstraction,  which  stands  in  the 
same  relation  to  generic,  as  generic  do  to  specific  abstrac- 
tions. Moreover,  the  definition  of  the  family  is  a  statement 
of  the  plan  of  all  the  genera  comprised  in  that  family. 

The  family  of  the  Northern  crayfishes  is  termed 
Potamobiidce ;  that  of  the  Southern  crayfishes,  Par- 
astacidce.  But  these  two  families  have  in  common  all 
those  structural  characters  which  are  special  to  neither  ; 
and,  carrying  out  the  metaphorical  nomenclature  of  the 
zoologist  a  stage  further,  we  may  say  that  the  two  form 
a  Tribe — the  definition  of  which  describes  the  plan  which 
is  common  to  both  families. 

It  may  conduce  to  intelligibility  if  these  results  are  put 
into  a  graphic  form.  In  fig.  66,  A.  is  a  diagram  represent 
ing  the  plan  of  an  animal  in  which  all  the  externally 
visible  parts  which  are  found,  more  or  less  modified,  in 
the  natural  objects  which  we  call  individual  cra3rfishes 
are  roughly  sketched.  It  represents  the  plan  of  the 
tribe.  B.  is  a  diagram  exhibiting  such  a  modification 
of  A.  as  converts  it  into  the  plan  common  to  the  whole 
family  of  the  Parastacidce.  C.  stands  in  the  same  re- 
lation to  the  Potamobiidce.  If  the  scheme  were  thoroughly 
worked  out,  diagrams  representing  the  peculiarities  of 


'254:  THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

form  which  characterize  each  of  the  genera  and  species, 
would  appear  in  the  place  of  the  names  of  the  former,  or 
of  the  circles  which  represent  the  latter.  All  these 
figures  would  represent  abstractions  —  mental  images 
which  have  no  existence  outside  the  mind.  Actual  facts 
would  begin  with  drawings  of  individual  animals,  which 
we  may  suppose  to  occupy  the  place  of  the  dots  above 
the  upper  line  in  the  diagram. 

That  all  crayfishes  may  be  regarded  as  modifications  of 
the  common  plan  A,  is  not  an  hypothesis,  but  a  generali- 
zation obtained  by  comparing  together  the  observations 
made  upon  the  structure  of  individual  crayfishes.  It  is 
simply  a  graphic  method  of  representing  the  facts  which 
are  commonly  stated  in  the  form  of  a  definition  of  the 
tribe  of  crayfishes,  or  Astacina. 

This  definition  runs  as  follows  : — 

Multicellular  animals  provided  with  an  alimentary 
canal  and  with  a  chitinous  cuticular  exoskeleton;  with 
a  ganglionated  central  nervous  system  traversed  by  the 
oesophagus  ;  possessing  a  heart  and  branchial  respiratory 
organs. 

The  body  is  bilaterally  symmetrical,  and  consists  of 
twenty  metameres  (or  somites  and  their  appendages),  of 
which  six  are  associated  into  a  head,  eight  into  a  thorax, 
and  six  into  an  abdomen.  A  telson  is  attached  to  the 
last  abdominal  somite. 

The  somites  of  the  abdominal  region  are  all  free,  those 
of  the  head  and  thorax,  except  the  hindermost,  which  is 


DEFINITION   OF  THE  GROUP  ASTACINA.  255 

purtially  free,  are  united  into  a  cephalo thorax,  the  tergal 
wall  of  which  has  the  form  of  a  continuous  carapace. 
The  carapace  is  produced  in  front  into  a  rostrum,  at  the 
sides  into  branchiostegites. 

The  eyes  are  placed  at  the  ends  of  movable  stalks. 
The  antennules  are  terminated  by  two  filaments.  The 
exopodite  of  the  antenna  has  the  form  of  a  mobile  scale. 
The  mandible  has  a  palp.  The  first  and  second  maxilla 
are  foliaceous ;  the  second  being  provided  with  a  large 
scaphognathite.  There  are  three  pairs  of  maxillipedes, 
and  the  endopodites  of  the  third  pair  are  narrow  and 
elongated.  The  next  pair  of  thoracic  appendages  is  much 
larger  than  the  rest,  and  is  chelate,  as  are  the  two  fol- 
lowing pairs,  which  are  slender  ambulatory  limbs.  The 
hindmost  two  pairs  of  thoracic  appendages  are  ambu- 
latory limbs,  like  the  foregoing,  but  not  chelate.  The 
abdominal  appendages  are  small  swimmerets,  except  the 
sixth  pair,  which  are  very  large,  and  have  the  exopodite 
divided  by  a  transverse  joint. 

All  the  crayfishes  have  a  complex  gastric  armature. 
The  seven  anterior  thoracic  limbs  are  provided  with 
podobranchiae,  but  the  first  of  these  is  always  more  or 
less  completely  reduced  to  an  epipodite.  More  or  fewer 
arthrobranchiaB  always  exist.  Pleurobranchise  may  be 
present  or  absent. 

In  this  tribe  of  Astacina  there  are  two  families,  the 
Potamobiida  and  the  Parastacida ;  and  the  definition  of 
each  of  these  families  is  formed  by  superadding  to  the 


256  THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

definition  of  the  tribe  the  statement  of  the  special  pecu- 
liarities of  the  family. 

Thus,  the  Potamoliidce  are  those  Astacina  in  which 
the  podobranchise  of  the  second,  fourth,  fifth,  and  sixth 
thoracic  appendages  are  always  provided  with  a  plaited 
lamina,  and  that  of  the  first  is  an  epipodite  devoid  of 
branchial  filaments.  The  first  abdominal  somite  invari- 
ably bears  appendages  in  the  males,  and  usually  in  both 
sexes.  In  the  males  these  appendages  are  styliform,  and 
those  of  the  second  somite  are  always  peculiarly  modified. 
The  appendages  of  the  four  following  somites  are  rela- 
tively small.  The  telson  is  very  generally  divided  by  a 
transverse  incomplete  hinge.  None  of  the  branchial  fila- 
ments are  terminated  by  hooks ;  nor  are  any  of  the 
coxopoditic  setaB,  or  the  longer  seta?  of  the  podobranchiaB 
hooked,  though  hooked  tubercles  occur  on  the  stem  and 
on  the  larninaB  of  the  latter.  The  coxopoditic  setaB  are 
always  long  and  tortuous. 

In  the  Parastacidce,  on  the  other  hand,  the  podo- 
branchise  are  devoid  of  more  than  a  rudiment  of  a 
lamina,  though  the  stem  may  be  alate.  The  podo- 
branchia  of  the  first  maxillipede  has  the  form  of  an 
epipodite ;  but,  in  almost  all  cases,  it  bears  a  certain 
number  of  well  developed  branchial  filaments.  The  first 
abdominal  somite  possesses  no  appendages  in  either  sex : 
and  the  appendages  of  the  four  following  somites  are 
large.  The  telson  is  never  divided  by  a  transverse  hinge. 
More  or  fewer  of  the  branchial  filaments  of  the  podo 


ALLIES   OF  THE  CRAYFISH.  257 

branchiae  are  terminated  by  short  hooked  spines ;  and  the 
coxopoditic  setae,  as  well  as  those  which  beset  the  sterna 
of  the  podobranchise,  have  hooked  apices. 

The  definitions  of  the  genera  would  in  like  manner  be 
given  by  adding  the  distinctive  characters  of  each  genus 
to  the  definitions  of  the  family ;  and  those  of  the  species 
by  adding  its  character  to  those  of  the  genus.  But  at 
present  it  is  unnecessary  to  pursue  this  topic  further. 

There  are  no  other  inhabitants  of  the  fresh  waters,  or 
of  the  land,  which  could  be  mistaken  for  crayfishes  ;  but 
certain  marine  animals,  familiar  to  every  one,  are  so 
strikingly  similar  to  them,  that  one  of  these  was  formerly 
included  in  the  same  genus,  Astacus ;  while  another  is 
very  often  known  as  the  "  Sea-crayfish."  These  are  the 
"  Common  Lobster,"  the  "  Norway  Lobster,"  and  the 
"  Rock  Lobster  "  or  "  Spiny  Lobster." 

The  common  lobster  (Homarus  vulgaris,  fig.  67) 
presents  the  following  distinctive  characters.  The  last 
thoracic  somite  is  firmly  adherent  to  the  rest;  the  exo- 
podite  of  the  antenna  is  so  small  as  to  appear  like  a  mere 
movable  scale ;  all  the  abdominal  appendages  are  well 
developed  in  both  sexes ;  and,  in  the  males,  the  two  an- 
terior pairs  are  somewhat  like  those  of  the  male  Astacus, 
but  less  modified. 

The  principal  difference  from  the  Astacina  is  exhibited 
by  the  gills,  of  which  there  are  twenty  on  each  side; 

namely,  six  podobranchiae,  ten  arthrobranchiae,  and  four 
18 


Flo-.  67.     Homarnti  nilgai-is  (^  nat.  size). 


HOMARUS   AND   NEPHROPS. 


259 


fully  developed  pleurobranchiae.  Moreover,  the  bran- 
chial filaments  of  these  gills  are  much  stiffer  and  more 
closely  set  than  in  most  crayfishes.  But  the  most  im- 
portant distinction  is  presented  by  the  podobranchia?,  in 
which  the  stem  is,  as  it  were,  completely  split  into  two 
parts  longitudinally  (as  in  fig.  68,  B)  ;  one  half  (ep) 


FIG.  68.  Podobranchiae  of  A,  Puraxtacu*  ;  B.  Xeph roj)s ;  C.  Pal&mon.. 
A ,  C',  transverse  sections  of  A  and  C  respectively,  a,  point  of  attach- 
ment ;  al,  wing-like  expansion  of  the  stem  ;  b,  base  ;  Z»r,  branchial 
filaments  ;  ep,  epipodite  ;  /,  branchial  laminae  ;  pi,  plume  ;  sf,  stem. 

corresponding  with  the  lamina  of  the  crayfish  gill,  and  the 
other  (pi)  with  its  plume.  Hence  the  base  (b)  of  the 
podobranchia  bears  the  gill  in  front;  while,  behind,  it 
is  continued  into  a  broad  epipoditic  plate  (ep)  slightly 
folded  upon  itself  longitudinally  but  not  plaited,  as  in  the 
crayfish. 

The    Norway  Lobster  (Nephrops  norveyicus,   fig.   69) 


Fig.  G9.      Neplirops  norvegwus  (^  nat.  size). 


THE  ROCK  LOBSTER    (PALINURUS).  261 

resembles  the  lobster  in  those  respects  in  which  the  latter 
.  differs  from  the  crayfishes  :  but  the  antennary  squame  is 
large ;  and,  in  addition,  the  branchial  plume  of  the  podo- 
branchia  of  the  second  maxillipede  is  very  small  or  absent, 
so  that  the  total  number  of  functional  branchiae  is  reduced 
to  nineteen  on  each  side. 

These  two  genera,  Homarus  and  Nephrops,  therefore, 
represent  a  family,  Homarina,  constructed  upon  the 
same  common  plan  as  the  crayfishes,  but  differing  so 
far  from  the  Astacina  in  the  structure  of  the  branchiae 
and  in  some  other  points,  that  the  distinction  must  be 
expressed  by  putting  them  into  a  different  tribe.  It  is 
obvious  that  the  special  characteristics  of  the  plan  of  the 
Homarina  give  it  much  more  likeness  to  that  of  the 
PotamoUidce  than  to  that  of  the  Parastacida. 

The  Eock  Lobster  (Palinurus,  fig.  70)  differs  much  more 
from  the  crayfishes  than  either  the  common  lobster  or 
the  Norway  lobster  does.  Thus,  to  refer  only  to  the  more 
important  distinctions,  the  antennas  are  enormous ;  none 
of  the  five  posterior  pairs  of  thoracic  limbs  are  chelate, 
and  the  first  pair  are  not  so  large  in  proportion  to  the 
rest  as  in  the  crayfishes  and  lobsters.  The  posterior 
thoracic  sterna  are  very  broad,  not  comparatively  narrow, 
as  in  the  foregoing  genera.  There  are  no  appendages 
to  the  first  somite  of  the  abdomen  in  either  sex.  In 
this  respect,  it  is  curious  to  observe  that,  in  contradis- 
tinction from  the  Homarina,  the  Rock  Lobsters  are  more 
closely  allied  to  the  Parastacidce  than  to  the  Potamobiida. 


FlG.  70.     Palinmus  vnlgaris  (about  ^  nat.  size). 


TRICHOBRANCHLE.  263 

The  gills  are  similar  to  those  of  the  lobsters,  but  reach 
the  number  of  twenty-one  on  each  side. 

In  their  fundamental  structure  the  rock  lobsters  agree 
with  the  crayfishes ;  hence  the  plans  of  the  two  may  be 
regarded  as  modifications  of  a  plan  common  to  both. 
To  this  end,  the  only  considerable  changes  needful  in 
the  tribal  plan  of  the  crayfishes,  are  the  substitution  of 
simple  for  chelate  terminations  to  the  middle  thoracic 
limbs  and  the  suppression  of  the  appendages  of  the  first 
somite  of  the  abdomen. 

Thus  not  only  all  the  crayfishes,  but  all  the  lobsters 
and  rock  lobsters,  different  as  they  are  in  appearance, 
size,  and  habits  of  life,  reveal  to  the  morphologist  un- 
mistakable signs  of  a  fundamental  unity  of  organization ; 
each  is  a  comparatively  simple  variation  of  the  general 
theme — the  common  plan. 

Even  the  branchiae,  which  vary  so  much  in  number  in 
different  members  of  these  groups,  are  constructed  upon 
a  uniform  principle,  and  the  differences  which  they 
present  are  readily  intelligible  as  the  result  of  various 
modifications  of  one  and  the  same  primitive  arrange- 
ment. 

In  all,  the  gills  are  trichobranchice  ;  that  is,  each  gill 
is  somewhat  like  a  bottle-brush,  and  presents  a  stem 
beset,  more  or  less  closely,  with  many  series  of  bran- 
chial filaments.  The  largest  number  of  complete  bran- 
chiae possessed  by  any  of  the  Potamobiidce,  Parastacidce, 
Hvmarida,  or  Palinuridce,  is  twenty-one  on  each  side ; 


264-  THE  COMPAKATIVE  MORPHOLOGY  OF  THE  CRAWFISH. 

and  when  this  number  is  present,  the  total  is  made  up 
of  the  same  numbers  of  podobranchise,  arthrobran- 
chii«,  and  pleurobranchiae  attached  to  corresponding 
somites.  In  Palinurus  and  in  the  genus  Astacopsis 
(which  is  one  of  the  Parastacidce),  for  example,  there  are 
six  podobranchise  attached  to  the  thoracic  limbs  from 
the  second  to  the  seventh  inclusively ;  five  pairs  of  artaro- 
branchiae  are  attached  to  the  interarticular  membranes 
of  the  thoracic  limbs  from  the  third  to  the  seventh 
inclusively,  and  one  to  that  of  the  second,  making  eleven 
in  all ;  while  four  pleurobranchise  are  fixed  to  the 
epimera  of  the  four  hindmost  thoracic  somites.  More- 
over, in  Astacopsis,  the  epipodite  of  the  first  thoracic 
appendage  (the  first  maxillipede)  bears  branchial  fila- 
ments, and  is  a  sort  of  reduced  gill. 

These  facts   may  be    stated    in    a    tabular  form   as 
follows : — 

The  branchial  formula  of  Astacopsis. 


Somites  and 
their 
Appendages. 

Arthrobranchise. 
Podo-                  •*•  —  v               PleiiBO- 
brauchise.       Anterior.      Posterior,      branchiae. 

VII.      ... 
VIII.      .  .. 

0  (ep.  r.) 
1 

0 
1 

...       0       ... 
...      0      ... 

0       = 

0       =. 

0  (ep.  r.) 
2 

IX.      ... 

1 

1 

...     1     ... 

0       = 

3 

X.      ... 

1 

1 

..!     i     ... 

0       = 

3 

XI.      ... 

1 

1 

...     i     ... 

1     = 

4 

XII.      ... 

1 

1 

i 

1      -= 

4 

XIII.      ... 

1 

1 

i 

1 

4 

xtv.    ... 

0 

0 

0 

1      = 

1 

6  +  ep.  r.    +     G       +       5      -f       4       =     21  -r  ep.  r. 


BRANCHIAL   FORMULAE. 


265 


This  tabular  "  branchial  formula  "  exhibits  at  a  glance 
not  only  the  total  number  of  branchiae,  but  that  of  each 
kind  of  branchia ;  and  that  of  all  kinds  connected  with 
each  somite ;  and  it  further  indicates  that  the  podo- 
branchia  of  the  first  thoracic  somite  has  become  so  far 
modified,  that  it  is  represented  only  by  an  epipodite,  with 
branchial  filaments  scattered  upon  its  surface. 

In  Paliuurus,  these  branchial  filaments  are  absent  and 
the  branchial  formula  therefore  becomes — 


Somites  and 
their 
Appendages. 

ArthrobranchisB. 
Podo-                 ,  *  x                Plenro- 
branchise.       Anterior.      Posterior.        branchiae. 

VII.      ... 
VIII.      ... 

0  (ep.) 
1 

0 

1 

...       0 
...      0 

...       0 
...       0 

= 

0  (ep.) 
2 

IX.      ... 

1 

1 

1 

0 

:= 

3 

X.      ... 

1 

1 

...     1 

0 

= 

3 

XI.      ... 

1 

1 

...     1 

1 

m 

4 

XII.      ... 

1 

1 

1 

1 

m 

4 

XIII.      ... 

1 

1 

1 

\ 

=: 

4 

XIV.      , 

0 

0 

...      0 

1 

= 

1 

6  +  ep.     +      6 


+       4      =      21  +  ep. 


In  the  lobster,  the  solitary  arthrobranchia  of  the  eighth 
somite  disappears,  and  the  branchiae  are  reduced  to  twenty 
on  each  side. 

In  Astacus,  this  branchia  remains ;  but,  in  the  English 
crayfish,  the  most  anterior  of  the  pleurobranchiae  has 
vanished  and  mere  rudiments  of  the  two  next  remain. 
It  has  been  mentioned  that  other  Astaci  present  a 
rudiment  of  the  first  pleurobranchia. 


266     THE   COMPAKAT1VE  MORPHOLOGY   OF  THE   CRAYFISH. 


The  branchial  formula  of  Astacus. 


Somites  and  Arthrobranchiae. 

their  Podo-          ., * „          Pleuro- 

Appendages.  branchiae.  Anterior.  Posterior,  branchiae. 


VII.      ...       ( 

)(ep.)...  ( 

)       ...  0 

...     0 

»  0  (ep.) 

VIII.      ... 

... 

...  0 

...      0 

=  2 

IX,     ... 

...  1 

o 

=  8 

X.     ... 

... 

...  1 

...     0 

-  3 

XI.      ... 

...  1 

...      0  or  r 

=  3  or  3  +  r 

XII.      ... 

... 

...  1 

r 

=  3  +  r 

XIII.      ... 

... 

...  1 

=  3  +  r 

XIV.     ...      ( 

)       ...  ( 

)      ...  0 

1 

=  1 

6+ep.  -h  6       +5     +       1+  2or3r=  18  +  ep.  +  2or3r. 

In  Cambarus,  the  number  of  the  branchiae  is  reduced 
to  seventeen  by  the  disappearance  of  the  last  pleuro- 
branchia ;  while,  in  Astacoides,  the  process  of  reduction 
is  carried  so  far,  that  only  twelve  complete  branchiae  are 
left,  the  rest  being  either  represented  by  mere  rudiments, 
or  disappearing  altogether. 

The  branchial  formula  of  Astacoides. 


Somites  and                               Arthrobranchiae. 
their                 Podo-             „  «  N            Pleuro- 

/Lppendages.      branchiae.  Anterior.     Posterior.      branchiae. 

vii.    ...    0  (ep.  r.)        0 

...       0       ... 

0 

= 

0  (ep.  r.) 

VIII.     ... 

r 

...       0       ... 

0 

= 

1  +  r 

IX.   ... 

I 

...      0      ... 

0 

= 

2 

X.     ... 

I 

r 

0 

= 

2  +  r 

XI.     ... 

I 

r 

0 

— 

2  +  r 

XII.     ... 

1 

r 

0 

= 

2  +  r 

XIII.     ...     1            ...           1 

r 

0 

= 

2  +  r 

XIV.    ...    0          ...         0 

...      0       ... 

1 

= 

1 

ep.  r       5  +  r  +  0  +  4  r  +  1       =     12  +  ep.  r.  +  5 1 . 


THE  BRANCHIAE  OF  PEN^US. 


267 


As  these  formulae  show,  those  trichobranchiate  crus- 
tacea,  which  possess  fewer  than  twenty-one  complete 
branchiae  on  each  side,  commonly  present  traces  of  the 
missing  ones,  either  in  the  shape  of  epipodites,  as  in  the 
case  of  the  podobranchiae,  or  of  minute  rudiments,  in  the 
case  of  the  arthrobranchise  and  the  pleurobranchiae. 

In  the  marine,  prawn-like,  genus  Penceus  (fig.  73, 
Chap.  VI.),  the  gills  are  curiously  modified  trichobran- 
chiae.  The  number  of  functional  branchiae  is,  as  in  the 
lobster,  twenty ;  but  the  study  of  their  disposition  shows 
that  the  total  is  made  up  in  a  very  different  way. 

The  branchial  formula  of  Penceus. 


Somites  and 

Arthrobranehise. 

their 

Podo-                 ..  *  „                Pleuro- 

Appendages. 

branchise.       Anterior.      Posterior.       branchiae. 

VII.      ... 

0  (ep.)      ... 

0       ..        0       = 

1  +  ep. 

VIII.      ... 

0  (ep.)      ... 

... 

= 

3  +  ep. 

IX.      ... 

0  (ep.)      ... 

... 

K 

3  +  ep. 

X.     „. 

0(ep.)     ... 

... 

= 

3  -l-  ep. 

XI.      ... 

0  (ep.)      ... 

... 

^ 

3  +  ep. 

XII.      ... 

0  (ep.)      ... 

... 

.. 

= 

3  +  ep. 

XIII.      ... 

0 

... 

.. 

=1 

3 

xrv. 

0              ...       0       ...       0       .. 

IS 

1 

0  +  6  ep.    +     7 


+       7      -      20  +  6  ep. 


This  case  is  very  interesting;  for  it  shows  that  the 
whole  of  the  podobranchiae  may  lose  their  branchial  charac- 
ter, arid  te  reduced  to  epipodites,  as  is  the  case  with  the 
first  in  the  crayfish  and  lobster,  and  indeed  in  most  of 
the  forms  under  consideration.  And  since  all  but  one  of 
the  somites  bear  both  arthrobranchiae  and  pleurobranohiae, 


268  THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

the  suggestion  arises  that  each  hypothetically  complete 
thoracic  somite  should  possess  four  gills  on  each  side, 
giving  the  following 

Hypothetically  complete  branchial  formula. 


Somites  and 

Arthrobranchise. 

their 

Podo-                     ,  A  ^                Plenro- 

Appendages. 

branchise.          Anterior.      Posterior.       branchiae. 

VII.      ... 

1 

1 

1        = 

4 

VIII.      ... 

1 

1 

.. 

1        = 

4 

IX.      ... 

1 

1 

1        = 

4 

X.      ... 

1 

1 

.. 

1        = 

4 

XI.      ... 

1 

1 

.. 

1        = 

4 

XII.      ... 

1 

1 

.. 

1        = 

4 

XIII.      ... 

1 

1 

.. 

1        = 

4 

XIV.      ... 

1                ...        1 

,. 

1        = 

4 

8          +          84-8       +       8      =      32 

Starting  from  this  hypothetically  complete  branchial 
formula,  we  may  regard  all  the  actual  formulae  as  pro- 
duced from  it  by  the  more  or  less  complete  suppression 
of  the  most  anterior,  or  of  the  most  posterior  branchiae, 
or  of  both,  in  each  series.  In  the  case  of  the  podo- 
branchise,  the  branchiae  are  converted  into  epipodites ;  in 
that  of  the  other  branchia3,  they  become  rudimentary,  or 
disappear. 

In  general  appearance  a  common  prawn  (Pal&mon, 
fig.  71)  is  very  similar  to  a  miniature  lobster  or  crayfish. 
Nor  does  a  closer  examination  fail  to  reveal  a  complete 
fundamental  likeness.  The  number  of  the  somites,  and 
of  the  appendages,  and  their  general  character  and  dispo- 


THE  PRAWNS.  269 

sitiun,  are  in  fact  the  same.  But,  in  the  prawn,  the  abdomen 
is  much  larger  in  proportion  to  the  cephalothorax ;   the 


B 


FIG.  71.     Pal&mon  jrrmaicpnfri*    (about  4  na-k    size).      A,  female; 
B,  filth  thoracic  appendage  of  male. 

basal  scale,  or  expedite  of  the  antenna,  is  much  larger ; 
the  external  maxillipedes  are  longer,  and  differless  from  the 
succeeding  thoracic  appendages.  The  first  pair  of  these, 
which  answers  to  the  forceps  of  the  crayfish,  is  chelate, 
but  it  is  very  slender ;  the  second  pair,  also  chelate,  is 
always  larger  than  the  first,  and  is  sometimes  exceedingly 


270   THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

long  and  strong  (fig.  71,  B) ;  the  remaining  thoracic 
limbs  are  terminated  by  simple  claws.  The  five  anterior 
Abdominal  somites  are  all  provided  with  large  swimmerets, 
which  are  used  like  paddles,  when  the  animal  swims 
quietly ;  and,  in  the  males,  the  first  pair  is  only  slightly 
different  from  the  rest.  The  rostrum  is  very  large,  and 
strongly  serrated. 

None  of  these  differences  from  the  crayfish,  however, 
is  so  great,  as  to  prepare  us  for  the  remarkable  change 
observable  in  the  respiratory  organs.  The  total  number 
of  the  gills  is  only  eight.  Of  these,  five  are  large  pleuro- 
branchise,  attached  to  the  epimera  of  the  five  hinder 
thoracic  somites  ;  two  are  arthrobranchise,  fixed  to  the 
interarticular  membrane  of  the  external  maxillipede ;  and 
one,  which  is  the  only  complete  podobranchia,  belongs 
to  the  second  maxillipede.  The  podobranchiae  of  the 
first  and  third  maxillipedes  are  represented  only  by  small 
epipodites.  The  branchial  formula  therefore  is  : — 


Somites  and 

Arthrobranchise. 

their 

Podo-                  .,  :->  N 

Pleuro- 

Appendages. 

branchiae.       Anterior.      Posterior. 

branchiae. 

VII.      ... 

0(ep.)             0      ...       0       ... 

0      =      0  (ep.) 

VIII.      ... 

1          ...           0       .  .       0       ... 

0      =       1 

IX.      ... 

0  (ep.)              1       . 

.     1     ... 

0      =      2  (ep.) 

X.      ... 

0 

0       . 

.       0       ... 

1       = 

XI.      ... 

0 

0       . 

.     o     ... 

1 

XII.      ... 

0 

0      . 

.       0       ... 

1       - 

XIII.      ... 

0 

0      . 

0      .. 

1       = 

XIV. 

0 

0 

0 

1       = 

1  +  2  ep.    +     1       +      1 


6       =       8  +  2epk 


PHYLLOBRANCHLffl.  271 

The  prawn,  in  fact,  presents  us  with  an  extreme  case 
of  that  kind  of  modification  of  the  branchial  system,  of 
which  Penceus  has  furnished  a  less  complete  example. 
The  series  of  the  podobranchi®  is  reduced  almost  to 
nothing,  while  the  large  pleurobranchiaB  are  the  chief 
organs  of  respiration. 

But  this  is  not  the  only  difference.  The  prawn's 
gills  are  not  brush-like,  but  are  foliaceous.  They  are 
not  tricJiobrandiicBy  but  pliyllobrancliia ;  that  is  to  say, 
the  central  stem  of  the  branchia,  instead  of  being  beset 
with  numerous  series  of  slender  filaments,  bears  only  two 
rows  of  broad  flat  lamellae  (fig.  68,  Q  C',  I),  which  are 
attached  to  opposite  sides  of  the  stem  (C',  *),  and  gradu- 
ally diminish  in  size  from  the  region  of  the  stem  by  which 
it  is  fixed,  upwards  and  downwards.  These  lamellae  are 
superimposed  closely  upon  one  another,  like  the  leaves  of 
a  book ;  and  the  blood  traversing  the  numerous  passages 
by  which  their  substance  is  excavated,  comes  into  close 
relation  with  the  currents  of  aerated  water,  which  are 
driven  between  the  branchial  leaflets  by  a  respiratory 
mechanism  of  the  same  nature  as  that  of  the  crayfish. 

Different  as  these  phyllobranchiaa  of  the  prawns  are  in 
appearance  from  the  trichobranchiffi  of  the  preceding 
Crustacea,  they  are  easily  reduced  to  the  same  type.  For  in 
the  genus  Axius,  which  is  closely  allied  to  the  lobsters, 
each  branchial  stem  bears  a  single  series  of  filaments  on  its 
opposite  sides ;  and  if  these  biserial  filaments  are  sup- 
posed to  widen  out  into  broad  leaflets,  the  transition  from 


272  THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH, 

the  trichobranchia  to   the   phyllobranchia  will  be  very 
easily  effected. 

The  shrimp  (Crangori)  also  possesses  phjdlobranchia^ 
and  differs  from  the  prawn  chiefly  in  the  character  of  its 
locomotive  and  prehensile  thoracic  limbs. 

There  are  yet  other  very  well-known  marine  animals, 
which,  in  common  appreciation,  are  rlways  associated  with 
the  lobsters  and  crayfishes,  although  the  difference  of 
general  appearance  is  vastly  greater  than  in  any  of  the 
cases  which  have  yet  been  considered.  These  are  the 
Crabs. 

In  all  the  forms  we  have  hitherto  been  considering, 
the  abdomen  is  as  long  as,  or  longer  than,  the  cephalo- 
thorax,  while  its  width  is  the  same,  or  but  little  less. 
The  sixth  somite  has  very  large  appendages,  which, 
together  with  the  telson,  make  up  a  powerful  tail-fin  ; 
and  the  large  abdomen  is  thus  fitted  for  playing  an 
important  part  in  locomotion. 

Again,  the  length  of  the  cephalothorax  is  much  greater 
than  its  width,  and  it  is  produced  in  front  into  a  long 
rostrum.  The  bases  of  the  antennae  are  freely  movable, 
and  they  are  provided  with  a  movable  exopodite.  More- 
over, the  eye- stalks  are  not  inclosed  in  a  cavity  or  orbit, 
and  the  eyes  themselves  appear  above  and  in  front  of 
the  antennules.  The  external  maxillipedes  are  narrow, 
and  their  endopodites  are  more  or  less  leg-like. 

None  of  these  statements  apply  to  the  crabs.     In  these 


THE  CEABS. 


273 


animals  the  abdomen  is  short,  flattened,  and  apt  to  escape 
immediate  notice,  as  it  is  habitually  kept  closely  applied 
against  the  under  surface  of  the  cephalothorax.  It  is 


FIG.  7'2.     rnncerpagr.rim,ma\e  (^  nat.  size).     A,  dorsal  view,  with  the 
abdoi_en  extended  ;   B,  front  view  of  "face."     as,  antennary  steinum  ; 
or,  orbit :    r,  rostrum  ;  1.  eyestalk  ;  2.  antennule  ;  3.  base  of  antenna ; 
3',  free  portion  of  antenna, 
19 


274   THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

not  used  as  a  swimming  organ ;  and  the  sixth  somite 
possesses  no  appendages  whatever.  The  hreadth  of  the 
oephalotborax  is  often  greater  than  its  length,  and  there 
io  no  prominent  rostrum.  In  its  place  there  is  a  trun- 
cated process  (fig.  72,  B,  r),  which  sends  down  a  vertical 
partition  and  divides  from  one  another  two  cavities,  in 
which  the  swollen  basal  joints  of  the  small  antennules  (£) 
are  lodged.  The  outer  boundary  of  each  of  these  cavities 
is  formed  by  the  basal  part  of  the  antenna  (5),  which  is 
firmly  fixed  to  the  edge  of  the  carapace.  There  is  no  exo- 
poditic  scale ;  and  the  free  part  of  the  antenna  (3')  is  very 
small.  The  convex  corneal  surface  of  the  eye  appears 
outside  the  base  of  the  antenna,  lodged  in  a  sort  of  orbit 
(or),  the  inner  margin  of  which  is  formed  by  the  base  of 
the  antenna,  while  the  upper  and  outer  boundaries  are 
constituted  by  the  carapace.  Thus,  while  in  all  the  pre- 
ceding forms,  the  eye  is  situated  nearest  the  middle  line, 
and  is  most  forward,  while  the  antennule  lies  outside 
and  behind  it,  and  the  antenna  comes  next ;  in  the  crab, 
the  antennule  occupies  the  innermost  place,  the  antenna 
comes  next,  and  the  eye  appears  to  be  external  to  and 
behind  the  other  two.  But  there  is  no  real  change  in 
the  attachments  of  the  eye-stalks.  For  if  the  antennule 
and  the  basal  joint  of  the  antenna  are  removed,  it  will  be 
seen  that  the  base  of  the  eye -stalk  is  attached,  as  in  the 
crayfish,  close  to  the  middle  line,  on  the  inner  side, 
and  in  front  of  the  antennule.  But  it  is  very  long  and 
extends  outwards,  behind  the  antennule  and  the  antenna ; 


THE  CRABS.  275 

its  corneal  surface  alone  being  visible,  as  it  projects  into 
the  orbit- 
Again,  the  ischiopodites  of  the  external  maxillipedes 
are  expanded  into  broad  quadrate  plates,  which  meet  in 
the  middle  line,  and  close  over  the  other  manducatory 
organs,  like  two  folding-doors  set  in  a  square  doorway. 
Behind  these  there  are  great  chelate  forceps,  as  in  the 
crayfish;  but  the  succeeding  four  pairs  of  ambulatory 
limbs  are  terminated  by  simple  claws. 

When  the  abdomen  is  forcibly  turned  back,  its  sternal 
surface  is  seen  to  be  soft  and  membranous.  There  are  no 
swimmerets ;  but,  in  the  female,  the  four  anterior  pairs 
of  abdominal  limbs  are  represented  by  singular  appen- 
dages, which  give  attachment  to  the  eggs ;  while  in  the 
males  there  are  two  pairs  of  styliform  organs  attached 
to  the  first  and  second  somites  of  the  abdomen,  which 
correspond  with  those  of  the  male  crayfishes. 

The  ventral  portions  of  the  branchiostegites  are 
sharply  bent  inwards,  and  their  edges  are  so  closely 
applied  throughout  the  greater  part  of  their  length  to 
the  bases  of  the  ambulatory  limbs,  that  no  branchial 
cleft  is  left.  In  front  of  the  bases  of  the  forceps,  how- 
ever, there  is  an  elongated  aperture,  which  can  be  shut 
or  opened  by  a  sort  of  valve,  connected  with  the  external 
maxillipede,  which  serves  for  the  entrance  of  water  into 
the  branchial  cavity.  The  water  employed  in  respiration, 
and  kept  in  constant  motion  by  the  action  of  the  sca- 
phognathite,  is  baled  out  through  two  aperturesj  which 


276      THE   COMPARATIVE   MORPHOLOGY   OF  THE  CRAYFISH, 

are  separated  from  the  foregoing  by  the  external  maxilli- 
pedes,  and  lie  at  the  sides  of  the  quadrate  space  ID 
which  these  organs  are  set. 

There  are  only  nine  gills  on  each  side,  and  these, 
as  in  the  prawn  and  shrimp,  are  phyllobranchise. 
Seven  of  the  branchiae  are  pyramidal  in  shape,  and  for 
the  most  part  of  large  size.  When  the  branchiostegite 
is  removed,  they  are  seen  lying  close  against  its  inner 
walls,  their  apices  converging  towards  its  summit.  The 
two  hindermost  of  these  gills  are  pleurobranchise,  the 
other  five  are  arthrobranchise.  The  two  remaining  gills 
are  podobranchiae,  and  belong  to  the  second  and  the 
third  maxillipedes  respectively.  Each  is  divided  into  a 
branchial  and  an  epipoditic  portion,  the  latter  having  the 
form  of  a  long  curved  blade.  The  branchial  portion  of 
the  podobranchia  of  the  second  maxillipede  is  long,  and 
lies  horizontally  under  the  bases  of  the  four  anterior 
arthrobranchise ;  while  the  gill  of  the  podobranchia  of 
the  third  maxillipede  is  short  and  triangular,  and  fits  in 
between  the  bases  of  the  second  and  the  third  arthro- 
branchise.  The  epipodite  of  the  third  maxillipede  is  very 
long,  and  its  base  furnishes  the  valve  of  the  afferent 
aperture  of  the  branchial  cavity,  which  has  been  men- 
tioned above.  The  podobranchia  of  the  first  maxillipede 
is  represented  only  by  a  long  curved  epipoditic  blade, 
which  can  sweep  over  the  outer  surface  of  the  gills,  and 
doubtless  serves  to  keep  them  clear  of  foreign  bodies. 


THE  BRANCHIAE  OF  THE  CRABS. 


277 


The  branchial  formula  of  Cancer  paguru*. 


Somites  and 
their                  Podo- 

Arthrobranchiae. 
„  *  x              Pleuro- 

Appendages.         branchiae. 

Anterior. 

Posterior        branchiae. 

VII. 

.    0(ep.) 

0       ... 

0 

0 

= 

0 

VIII. 

1 

1       ... 

0 

0 

as 

2 

IX 

.     1 

1 

1 

0 

= 

3 

X. 

0 

1 

1 

0 

ssr 

2 

XI. 

.     0 

0       ... 

0 

1 

= 

1 

XII. 

.     0 

0       ... 

0 

1 

= 

1 

XIII. 

.     0 

0       ... 

0       .. 

0 

= 

0 

XIV. 

.     0 

0       ... 

0 

0 

= 

0 

'   I 


2  +  ep.    +    3       +      2 


9  +  ep. 


It  will  be  observed  that  the  suppression  of  branchiae 
has  here  taken  place  in  all  the  series,  and  at  both  the 
anterior  and  the  posterior  ends  of  each.  But  the  defect 
in  total  number  is  made  up  by  the  increase  of  size,  not  of 
the  pleurobranchiae  alone,  as  in  the  case  of  the  prawns, 
but  of  the  arthrobranchiae  as  well.  At  the  same  time 
the  whole  apparatus  has  become  more  specialized  and 
perfected  as  a  breathing  organ.  The  close  fitting  of  the 
edges  of  the  carapace,  and  the  possibility  of  closing  the 
inhalcnt  and  exhalent  apertures,  render  the  crabs  much 
more  independent  of  actual  immersion  in  water  than  most 
of  their  congeners  ;  and  some  of  them  habitually  live  on 
dry  land  and  breathe  by  means  of  the  atmospheric  air 
which  they  take  into  and  expel  from  their  branchial  cavities. 

Notwithstanding  all  these  wide  departures  from  the 
structure  and  habits  of  the  crayfishes,  however,  attentive 
examination  shows  that  the  plan  of  construction  of  the 

COLLEGE    Of    DENTISTRY 
UNIVERSITY  OF  CALIFORNIA 


278   THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

crab  is,  in  all  fundamental  respects,  the  same  as  that  of  the 
crayfish.  The  hody  is  made  up  of  the  same  number  of 
somites.  The  appendages  of  the  head  and  of  the  thorax 
are  identical  in  number,  in  function,  and  even  in  the 
general  pattern  of  their  structure.  But  two  pairs  of 
abdominal  appendages  in  the  female,  and  four  pairs  in 
the  male,  have  disappeared.  The  exopodites  of  the 
antennae  have  vanished,  and  not  even  epipodites  re- 
main to  represent  the  podobranchise  of  the  posterior  five 
pairs  of  thoracic  limbs.  The  exceedingly  elongated  eye- 
stalks  are  turned  backwards  and  outwards,  above  the 
bases  of  the  antennules  and  the  antennae,  and  the  bases 
of  the  latter  have  become  united  with  the  edges  of  the 
carapace  in  front  of  them.  In  this  manner  the  extra- 
ordinary face,  or  metope  (fig.  72,  B)  of  the  crab  results 
from  a  simple  modification  of  the  arrangement  of  parts, 
every  one  of  which  exists  in  the  crayfish.  The  same 
common  plan  serves  for  both. 

The  foregoing  illustrations  are  taken  from  a  few  of  our 
commonest  and  most  easily  obtainable  Crustacea ;  but  they 
amply  suffice  to  exemplify  the  manner  in  which  the  con- 
ception of  a  plan  of  organization,  common  to  a  multitude 
of  animals  of  extremely  diverse  outward  forms  and  habits, 
is  forced  upon  us  by  mere  comparative  anatomy. 

Nothing  would  be  easier,  were  the  occasion  fitting,  than 
to  extend  this  method  of  comparison  to  the  whole  of  the 
several  thousand  species  of  crab -like,  crayfish-like,  or 


THE  CRUSTACEA.  279 

prawn-like  animals,  which,  from  the  fact  that  they  all 
have  their  eyes  set  upon  movable  stalks,  are  termed  the 
Podophthalmia,  or  stalk-eyed  Crustacea ;  and  by  aigu- 
m  cuts  of  similar  force  to  prove  that  they  are  all  modifica- 
tions of  the  same  common  plan.  Not  only  so,  but  the 
sand-hoppers  of  the  sea-shore,  the  wood-lice  of  the- land, 
and  the  water-fleas  or  the  monoculi  of  the  ponds,  nay, 
even  such  remote  forms  as  the  barnacles  which  adhere  to 
floating  wood,  and  the  acorn  shells  which  crowd  every  inch 
of  rock  on  many  of  our  coasts,  reveal  the  same  funda- 
mental organization.  Further  than  this,  the  spiders 
and  the  scorpions,  the  millipedes  and  the  centipedes,  and 
the  multitudinous  legions  of  the  insect  world,  show  us, 
amid  infinite  diversity  of  detail,  nothing  which  is  new  in 
principle  to  any  one  who  has  mastered  the  morphology 
of  the  crayfish. 

Given  a  body  divided  into  somites,  each  with  a  pair 
of  appendages;  and  given  the  power  to  modify  those 
somites  and  their  appendages  in  strict  accordance  with 
the  principles  by  which  the  common  plan  of  the  Podoph- 
thalmia is  modified  in  the  actually  existing  members  of 
that  order ;  and  the  whole  of  the  Arthropoda  t  which 
probably  make  up  two-thirds  of  the  animal  world,  might 
readily  be  educed  from  one  primitive  form. 

And  this  conclusion  is  not  merely  speculative.  As  a 
matter  of  observation,  though  the  Arthropoda  are  not  all 
evolved  from  one  primitive  form,  in  one  sense  of  the 
words,  yet  they  are  in  another.  For  each  can  be  traced 


280   THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

back  in  the  course  of  its  development  to  an  ovum,  and 
that  ovum  gives  rise  to  a  blastoderm,  from  which  the 
parts  of  the  embryo  arise  in  a  manner  essentially  similar 
to  that  in  which  the  young  crayfish  is  developed. 

Moreover,  in  a  large  proportion  of  the  Crustacea,  the 
embryo  leaves  the  egg  under  the  form  of  a  small  oval 
body,  termed  a  Nauplius  (fig.  73,  D),  provided  with 
(usual!}7)  three  pairs  of  appendages,  which  play  the  part 
of  swimming  limbs,  and  with  a  median  eye.  Changes  of 
form  accompanied  by  sheddings  of  the  cuticle  take  place, 
in  virtue  of  which  the  larva  passes  into  a  new  stage,  when 
it  is  termed  a  Zocea  (C).  In  this,  the  three  pairs  of  loco- 
motive appendages  of  the  Nauplius  are  metamorphosed 
into  rudimentary  antennules,  antennae,  and  mandibles, 
while  two  or  more  pairs  of  anterior  thoracic  appendages 
provided  with  exopodites  and  hence  appearing  bifurcated, 
subserve  locomotion.  The  abdomen  has  grown  out  and 
become  a  notable  feature  of  the  Zoaea,  but  it  has  no 
appendages. 

In  some  Podophthalmia,  as  in  Pen&us  (fig.  73),  the 
young  leaves  the  egg  as  a  Nauplius,  and  the  Nauplius 
becomes  a  Zosea.  The  hinder  thoracic  appendages,  each 
provided  with  an  epipodite,  appear ;  the  stalked  eyes  and 
the  abdominal  members  are  developed,  and  the  larva  passes 
into  what  is  sometimes  called  the  My  sis  or  Schizopod 
stage.  The  adult  state  differs  from  this  chiefly  in  the 
presence  of  branchiae  and  the  rudimentary  character  of 
the  exopodites  of  the  five  posterior  thoracic  limbs. 


METAMORPHOSES  OF   THE   CRUSTACEA. 


281 


In    the    Opossum-shrimps    (Mysis)   the    young    does 
not   leave   the  pouch   of  the   mother    until    it  is   fully 


FIG.  73.  Penteus  semuulcatus.  A,  adult  (after  deHaan.  \  nat.  size)  ; 
B,  Zo£ea,  and  C,  less  advanced  Zosea  of  a  species  of  Pencem.  D, 
Nauplius.  (B,  C,  and  D,  after  Fritz  Muller.) 


282   THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

developed;  and,  in  this  case,  the  Xan^lim  state  is 
passed  through  so  rapidly  and  in  so  early  and  imperfect  a 
condition  of  the  embryo,  that  it  would  not  be  recognized 


FIG.  74.  Cancer  pagurus.  A,  newly  hatched  Zoaea  ;  B,  more  advanced 
Zosea  ;  C,  dorsal,  and  D,  side  view  of  Megalopa  (after  Spence  Bate). 
The  figures  A  and  B  are  more  magnified  than  C  and  D.) 

except  for  the  cuticle  which  is  developed   and  is   subse- 
quently shed. 


METAMORPHOSES  OF  THE  CRUSTACEA.  283 

In  the  great  majority  of  the  Podophthalmia',  the  Nauplius 
stage  seems  to  be  passed  over  without  any  such  clear 
evidence  of  its  occurrence,  and  the  young  is  set  free  as  a 
Zoaea.  In  the  lobsters,  which  have,  throughout  life,  a 
large  abdomen  provided  with  swimmerets,  the  Zosea, 
after  going  through  a  My  sis  or  Schizopod  stage,  passes 
into  the  adult  form. 

In  the  crab,  the  young  leaves  the  egg  as  a  Zoaa 
(fig.  74,  A  and  B).  But  this  is  not  followed  by  a 
Schizopod  stage,  inasmuch  as  the  five  hinder  pair  of 
thoracic  limbs  are  apparently,  from  the  first,  devoid  of 
exopodites.  But  the  Zoaea,  after  it  has  acquired  stalked 
eyes  and  a  complete  set  of  thoracic  and  abdominal 
members,  and  has  passed  into  what  is  called  the  Mega- 
lopa  stage  (fig.  74,  C  and  D),  suffers  a  more  complete 
metamorphosis.  The  carapace  widens,  the  fore  part  of 
the  head  is  modified  so  as  to  bring  about  the  formation 
of  the  characteristic  metope  :  and  the  abdomen,  losing 
more  or  fewer  of  its  posterior  appendages,  takes  up  its 
final  position  under  the  thorax. 

In  the  Zosea  state,  those  thoracic  limbs  which  give  rise 
to  the  maxillipedes  are  provided  with  well-developed 
exopodites,  and  in  the  free  Mysis  state  all  these  limbs 
have  exopodites.  In  the  Opossum -shrimps  these  persist 
throughout  life ;  in  Penceus,  the  rudiments  of  them  only 
remain ;  in  the  lobster,  they  disappear  altogether. 

Thus,  in  these  animals,  there  is  no  difficulty  in  demon- 
strating that  embryological  uniformity  of  type  of  all  the 


284  THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

limbs,  complete  evidence  of  which  was  not  furnished  by  the 
development  of  the  crayfish.  In  this  crustacean,  in  fact, 
it  would  appear  that  the  process  of  development  has 
undergone  its  maximum  of  abbreviation .  The  embryo 
presents  no  distinct  and  independent  Nauplius  or  Zo&a 
stages,  and,  as  in  the  crab,  there  is  no  Schizopod  or 
Mysis  stage.  The  abdominal  appendages  are  developed 
very  early,  and  the  new  born  young,  which  resembles  the 
Megalopa  stage  of  the  crab,  differs  only  in  a  few  points 
from  the  adult  animal. 

Guided  by  comparative  morphology,  we  are  thus  led 
to  admit  that  the  whole  of  the  Arthropoda  are  connected 
by  closer  or  more  remote  degrees  of  affinity  with  the 
crayfish.  If  we  were  to  study  the  perch  and  the  pond- 
snail  with  similar  care,  we  should  be  led  to  analogous 
conclusions.  For  the  perch  is  related  by  similar  grada- 
tions, in  the  first  place,  with  other  fishes ;  then  more 
remotely,  with  frogs  and  newts,  reptiles,  birds,  and 
mammals ;  or,  in  other  words,  with  the  whole  of  the 
great  division  of  the  Vertebrata.  The  pond -snail,  by 
like  reasoning  upon  analogous  data,  is  connected  with 
the  Mollusca,  in  all  their  innumerable  kinds  of  slugs, 
shellfish,  squids,  and  cuttlefish.  And,  in  each  case,  the 
study  of  development  takes  us  back  to  an  egg  as  the 
primary  condition  of  the  animal,  and  to  the  process  of 
yelk  division,  the  formation  of  a  blastoderm,  and  the  con- 
version of  that  blastoderm  into  a  more  or  less  modified 


THE  COMMON   PLAN   OF   ANIMALS.  285 

gastrula,  as  the  early  stages  of  development.  The  like 
is  true  of  all  the  worms,  sea-urchins,  starfishes,  jellyfishes, 
polypes,  and  sponges ;  and  it  is  only  in  the  minutest  and 
simplest  forms  of  animal  life  that  the  germ,  or  repre- 
sentative of  the  ovum  becomes  metamorphosed  into  the 
adult  form  without  the  preliminary  process  of  division. 

In  the  majority  even  of  these  Protozoa,  the  typical 
structure  of  the  nucleated  cell  is  retained,  and  the  whole 
animal  is  the  equivalent  of  a  histological  unit  of  one  of 
the  higher  organisms.  An  Amc?ba  is  strictly  comparable, 
morphologically,  to  one  of  the  corpuscles  of  the  blood  of 
the  crayfish. 

Thus,  to  exactly  the  same  extent  as  it  is  legitimate 
to  represent  all  the  crayfishes  as  modifications  of  the 
common  astacine  plan,  it  is  legitimate  to  represent  all 
the  multicellular  animals  as  modifications  of  the  gastrula, 
and  the  gastrula  itself  as  a  peculiarly  disposed  aggregate 
of  cells ;  while  the  Protozoa  are  such  cells  either  isolated, 
or  otherwise  aggregated. 

It  is  easy  to  demonstrate  that  all  plants  .are  either 
cell  aggregates,  or  simple  cells ;  and  as  it  is  impossible 
to  draw  any  precise  line  of  demarcation,  either  physio- 
logical or  morphological,  between  the  simplest  plants, 
and  the  simplest  of  the  Protozoa,  it  follows  that  all  forms 
of  life  are  morphologically  related  to  one  another ;  and 
that  in  whatever  sense  we  say  that  the  English  and  the 
Californian  crayfish  are  allied,  in  the  same  sense,  though 
not  to  the  same  degree,  must  we  admit  that  all  living  things 


286   THE  COMPARATIVE  MORPHOLOGY  OF  THE  CRAYFISH. 

are  allied.  'Given  one  of  those  protoplasmic  bodies,  of 
which  we  are  unable  to  say  certainty  whether  it  is  animal 
or  plant,  and  endow  it  with  such  inherent  capacities  of 
Belf-modification  as  are  manifested  daily  under  our  eyes 
by  developing  ova,  and  we  have  a  sufficient  reason  for 
the  existence  of  any  plant,  or  of  any  animal. 

This  is  the  great  result  of  comparative  morphology; 
and  it  is  carefully  to  be  noted  that  this  result  is  not  a 
speculation,  but  a  generalisation.  The  truths  of  anatomy 
and  of  embryology  are  generalised  statements  of  facts 
of  experience  ;  the  question  whether  an  animal  is  more 
or  less  like  another  in  its  structure  and  in  its  develop- 
ment, or  not,  is  capable  of  being  tested  by  observation  ; 
the  doctrine  of  the  unity  of  organisation  of  plants  and 
animals  is  simply  a  mode  of  stating  the  conclusions 
drawn  from  experience.  But,  if  it  is  a  just  mode  of 
stating  these  conclusions,  then  it  is  undoubtedly  con- 
ceivable that  all  plants  and  all  animals  may  have  been 
evolved  from  a  common  physical  basis  of  life,  by  pro- 
cesses similar  to  those  which  we  every  day  see  at  work 
in  the  evolution  of  individual  animals  and  plants  from 
that  foundation.  t 

That  which  is  conceivable,  however,  is  by  no  means 
necessarily  true ;  and  no  amount  of  purely  morpho- 
logical evidence  can  suffice  to  prove  that  the  forms 
of  life  have  come  into  existence  in  one  way  rather 
than  another. 

There  is  a  common  plan  among  churches,  no  less  than 


THE  MORPHOLOGICAL   UNITY   OF   LIVING  THINGS.      287 

among  crayfishes  ;  nevertheless  the  churches  have  cer- 
tainty not  been  developed  from  a  common  ancestor,  but 
have  been  built  separately.  Whether  the  different  kinds 
of  crayfishes  have  been  built  separately,  is  a  problem  we 
shall  not  be  in  a  position  to  grapple  with,  until  we  have 
considered  a  series  of  facts  connected  with  them,  which 
have  not  yet  been  touched  upon. 


CHAPTER  VL 

THE    DISTRIBUTION    AND    THE    .ETIOLOGY    OP  THK 
CRAYFISHES. 

So  far  as  I  have  been  able  to  discover,  all  the  cray- 
fishes which  inhabit  the  British  islands  agree  in  every 
point  with  the  full  description  given  above,  at  p.  230. 
They  are  abundant  in  some  of  our  rivers,  such  as  the 
Isis,  and  other  affluents  of  the  Thames ;  and  they  have 
been  observed  in  those  of  Devon ;  *  but  they  appear  to 
be  absent  from  many  others.  I  cannot  hear  of  any,  for 
example,  in  the  Cam  or  the  Ouse,  on  the  east,  or  in 
the  rivers  of  Lancashire  and  Cheshire,  on  the  west. 
It  is  still  more  remarkable  that,  according  to  the  best 
information  I  can  obtain,  they  are  absent  in  the  Severn, 
though  they  are  plentiful  in  the  Thames  and  Severn  canal. 
Dr.  MMntosh,  who  has  paid  particular  attention  to  the 
fauna  of  Scotland,  assures  me  that  crayfish  are  unknown 
north  of  the  Tweed.  In  Ireland,  on  the  other  hand, 
they  occur  in  many  localities  ;  t  but  the  question  whether 
their  diffusion,  and  even  their  introduction  into  this 

*  Moore.     Magazine  of  Natural  History.     New  Series,  III.,  1839. 
t  Thompson.     Annals  and  Magazine  of  Natural  History,  XI.,  1843. 


THE  NAME  ASTACUS  FLUVIATILIS.  289 

island,  has  or  has  not  been  effected  by  artificial  means,  is 
involved  in  some  obscurity. 

English  zoologists  have  always  termed  our  crayfish 
Astacus  fluiiatilis ;  and,  up  to  a  recent  period,  the 
majority  of  Continental  naturalists  have  included  a 
corresponding  form  of  Astacus  under  that  specific  name. 

Thus  M.  Milne  Edwards,  in  his  classical  work  on  the 
Crustacea*  published  in  1837,  observes  under  the  head  of 
"  Ecrevisse  commune.  Astacus  fluviatilis  :"  "  There  are 
two  varieties  of  this  crayfish;  in  the  one,  the  rostrum 
gradually  becomes  narrower  from  its  base  onwards,  and 
the  lateral  spines  are  situated  close  to  its  extremity; 
in  the  other,  the  lateral  edges  of  the  rostrum  are  parallel 
in  their  posterior  half  and  the  lateral  spines  are  stronger 
and  more  remote  from  the  end." 

The  "  first  variety,"  here  mentioned,  is  known  under 
the  name  of  "  Ecrevisse  a  pieds  blancs "  f  in  France, 
by  way  of  distinction  from  the  "  second  variety,"  which 
is  termed  "Ecrevisse  a  pieds  rouges,"  on  account  of 
the  more  or  less  extensive  red  coloration  of  the  forceps 
and  ambulatory  limbs.  This  second  variety  is  the  larger, 
commonly  attaining  five  inches  in  length,  and  sometimes 
reaching  much  larger  dimensions ;  and  it  is  more  highly 
esteemed  for  the  market,  on  account  of  its  better  flavour. 
In  Germany,  the  two  forms  have  long  been  popularly 
distinguished,  the  former  by  the  name  of  "  Steinkrebs," 

*  "  Histoire  Naturelle  des  Crustacea." 
f  Carbonnier.     "L'Ecrevisse,"  p.  8. 
20 


290      DISTRIBUTION  AND  AETIOLOGY   OF  THE  CRAYFISHES 

or  "  stone  crayfish,"  and  the  latter  by  that  of  "  Edel- 
krebs,"  or  "  noble  crayfish." 

Milne  Edwards,  it  will  be  observed,  speaks  of  these 
two  forms  of  crayfish  as  "  varieties "  of  the  species 
Astacus  fluviatilis;  but,  even  as  far  back  as  the  year 
1803  some  zoologists  began  to  regard  the  "  stone  cray- 
fish "  as  a  distinct  species,  to  which  Schrank  applied  the 
name  of  Astacus  torrentium,  while  the  "  noble  craj^fish  " 
remained  in  possession  of  the  old  denomination,  Astacus 
fluviatilis ;  and,  subsequently,  various  forms  of  "  stone- 
crayfishes  "  have  been  further  distinguished  as  the  species 
Astacus  saxatilis,  A.  tristis,  A.  pallipes,  A.  fontinalis, 
&c.  On  the  other  hand,  Dr.  Gerstfeldt,*  who  has  devoted 
especial  attention  to  the  question,  denies  that  these 
are  anything  more  than  varieties  of  one  species ;  but  he 
holds  this  and  Milne  Edwards's  "  second  variety  "  to  be 
specifically  distinct  from  one  another. 

We  thus  find  ourselves  in  the  presence  of  three  views 
respecting  the  English  and  French  crayfishes. 

1.  They  are  all  varieties  of  one  species — A.  fluviatilis. 

2.  There   are  two  species — A.  fluviatilis,  and  A.  tor- 
rentium,  of  which  last  there  are  several  varieties. 

3.  There  are,  at  fewest,  five  or  six  distinct  species. 
Before    adopting    the    one    or    the    other    of    these 

views,  it  is  necessary  to  form  a  definite  conception    of 
the  meaning  of  the  terms  "  species  "  and  "  variety." 

*  "  Uebei  die  Flusskrebse  Europas."  Mem.  de  TAcad.  de  St.  Peters- 
burg,  1859. 


THE  MEANING  OF  THE  WORD  SPECIES.  291 

The  word  "  species  "  in  Biology  has  two  significations  ; 
the  one  based  upon  morphological,  the  other  upon 
physiological  considerations. 

A  species,  in  the  strictly  morphological  sense,  is  simply 
an  assemblage  of  individuals  which  agree  with  one  another, 
and  differ  from  the  rest  of  the  living  world  in  the  sum 
of  their  morphological  characters ;  that  is  to  say,  in 
the  structure  and  in  the  development  of  both  sexes. 
If  the  sum  of  these  characters  in  one  group  is  repre- 
sented by  A,  and  that  in  another  by  A  -f-  n ;  the  two 
are  morphological  species,  whether  n  represents  an 
important  or  an  unimportant  difference. 

The  great  majority  of  species  described  in  works  on 
Systematic  Zoology  are  merely  morphological  species. 
That  is  to  say,  one  or  more  specimens  of  a  kind  of  animal 
having  been  obtained,  these  specimens  have  been  found 
to  differ  from  any  previously  known  by  the  character  or 
characters  n;  and  this  difference  constitutes  the  defi- 
nition of  the  new  species,  and  is  all  we  really  know 
about  its  distinctness. 

But,  in  practice,  the  formation  of  specific  groups  is 
more  or  less  qualified  by  considerations  based  upon  what 
is  known  respecting  variation.  Ic  is  a  matter  of  obser- 
vation that  progeny  are  never  exactly  like  their  parents, 
but  present  small  and  inconstant  differences  from  them. 
Hei-'Ce,  when  specific  identity  is  predicated  of  a  group  of 
individuals,  the  meaning  conveyed  is  not  that  they  are 
all  exactly  alike,  but  only  that  their  differences  are  so 


292      DISTRIBUTION  AND  .ETIOLOGY  OF  THE  CRAYFISHES. 

small,   and    so     inconstant,    that    they  lie   within    the 
probable  limits  of  individual  variation. 

Observation  further  acquaints  us  with  the  fact,  that, 
sometimes,  an  individual  member  of  a  species  may 
exhibit  a  more  or  less  marked  variation,  which  is  pro 
pagated  through  all  the  offspring  of  that  individual, 
and  may  even  become  intensified  in  them.  And,  in 
this  manner,  a  variety,  or  race,  is  generated  within  the 
species;  which  variety,  or  race,  if  nothing  were  known 
respecting  its  origin,  might  have  every  claim  to  be 
regarded  as  a  separate  morphological  species.  Tl.o 
distinctive  characters,  of  a  race,  however,  are  raivly 
equally  well  marked  in  all  the  members  of  the  racp. 
Thus  suppose  the  species  A  to  develope  the  race  A  -f  x  ; 
then  the  difference  x  is  apt  to  be  much  less  in  some 
individuals  than  in  others ;  so  that,  in  a  large  suite  of 
specimens,  the  interval  between  A  +  x  and  A  will  be 
filled  up  by  a  series  of  forms  in  which  x  gradually 
diminishes. 

Finally,  it  is  a  matter  of  observation  that  modification 
of  the  physical  conditions  under  which  a  species  lives 
favours  the  development  of  varieties  and  races. 

Hence,  in  the  case  of  two  specimens  having  respec- 
tively the  characters  A  and  A  +  n,  although,  primd  facie, 
they  are  of  distinct  species  ;  yet  if  a  large  collection 
shows  us  that  the  interval  between  A  and  A  +  n  is  filled 
up  by  forms  of  A  having  traces  of  n,  and  forms  of  A  +  n 
in  which  n  becomes  less  and  less,  then  it  will  be  con- 


VAKIETIES  AND  TRANSITIONAL  FORMS.  293 

eluded  that  A  and  A  +  n  are  races  of  one  species  and 
not  separate  species.  And  this  conclusion  will  be  fortified 
if  A  and  A  +  n  occupy  different  stations  in  the  same 
geographical  area. 

Even  when  no  transitional  forms  between  A  and  A  +  n 
are  discoverable,  if  n  is  a  small  and  unimportant  differ- 
ence, such  as  of  average  size,  colour,  or  ornamenta- 
tion, it  may  be  fairly  held  that  A  and  A  +  n  are  mere 
varieties ;  inasmuch  as  experience  proves  that  such 
variations  may  take  place  comparatively  suddenly;  or 
the  intermediate  forms  may  have  died  out  and  thus  the 
evidence  of  variation  may  have  been  effaced. 

From  what  has  been  said  it  follows  that  the  groups 
termed  morphological  species  are  provisional  arrange- 
ments, expressive  simply  of  the  present  state  of  our 
knowledge. 

We  call  two  groups  species,  if  we  know  of  no  tran- 
sitional forms  between  them,  and  if  there  is  no  reason  to 
believe  that  the  differences  which  they  present  are  such 
as  may  arise  in  the  ordinary  course  of  variation.  But 
it  is  impossible  to  say  whether  the  progress  of  in- 
quiry into  the  characters  of  any  group  of  individuals 
may.  prove  that  what  have  hitherto  been  taken  for  mere 
varieties  are  distinct  morphological  species ;  or  whether, 
on  the  contrary,  it  may  prove  that  what  have  hitherto 
been  regarded  as  distinct  morphological  species  are  mere 
varieties. 

Wh&t  has  happened  in  the  case  of  the  crayfish  is  this : 


294      DISTRIBUTION  AND   AETIOLOGY   OF  THE   CRAYFISHES. 

the  older  observers  lumped  all  the  Western  European 
forms  which  came  under  their  notice  under  one  species, 
Astacus  fluviatilis ;  noting,  more  or  less  distinctly,  the 
stone  crayfish  and  the  noble  crayfish  as  races  or  varieties 
of  that  species.  Later  zoologists,  comparing  crayfishes 
together  more  critically,  and  finding  that  the  stone 
crayfish  is  ordinarily  markedly  different  from  the  noble 
crayfish,  concluded  that  there  were  no  transitional  forms, 
and  made  the  former  into  a  distinct  species,  tacitly  as- 
suming that  the  differential  characters  are  not  such  as 
could  be  produced  by  variation. 

It  is  at  present  an  open  question  whether  further 
investigation  will  or  will  not  bear  out  either  of  these 
assumptions.  If  large  series  of  specimens  of  both  stone 
crayfishes  and  noble  crayfishes  from  different  localities 
are  carefully  examined,  they  will  be  found  to  present 
great  variations  in  size  and  colour,  in  the  tuberculation 
of  the  carapace  and  limbs,  and  in  the  absolute  and 
relative  sizes  of  the  forceps. 

The  most  constant  characters  of  the  stone  crayfish 
are  : — 

1.  The  tapering  form  of  the  rostrum  and  the  approxi- 
mation of  the  lateral  spines  to  its  point ;  the  distance 
between  these  spines  being  about  equal  to  their  distance 
from  the  apex  of  the  rostrum  (fig.  61,  A). 

2.  The  development   of  one  or  two  spines  from  the 
ventral  margin  of  the  rostrum. 

3.  The  gradual  subsidence  of   the  posterior  part   of 


THE  STONE  CRAYFISH  AND  THE  NOBLE  CRAYFISH.   295 

the  post- orbital  ridge,  and  the  absence  of  spines  on  its 
surface. 

4.  The  large  relative  size  of  the  posterior  division  of 
the  telson  (G). 

On  the  contrary,  in  the  noble  crawfish  : — 

1 .  The    sides    of  the    posterior    two  -  thirds    of   the 
rostrum  are  nearl}7  parallel,   and  the  lateral  spines  are 
fulty  a  third  of  the  length  of  the  rostrum  from  its  point ; 
the  distance  between  them  being  much  less  than  their 
distance  from  the  apex  of  the  rostrum  (B). 

2.  No  spine  is  developed  from  the  ventral  margin  of 
the  rostrum. 

3.  The  posterior  part  of  the  post-orbital  ridge  is  a 
more  or  less  distinct,  sometimes  spinous  elevation. 

4.  The   posterior   division   of   the    telson   is   smaller 
relatively  to  the  anterior  division  (H). 

I  may  add  that  I  have  found  three  rudimentary  pleuro- 
branchiaB  in  the  noble  crayfish,  and  never  more  than  two 
in  the  stone  crayfish. 

In  order  to  ascertain  whether  no  crayfish  exist  in 
which  the  characters  of  the  parts  here  referred  to  are 
intermediate  between  those  defined,  it  would  be  neces- 
sary to  examine  numerous  examples  of  each  kind  of  cray- 
fish from  all  parts  of  the  areas  which  they  respectively 
inhabit.  This  has  been  done  to  some  extent,  but  by  no 
means  thoroughly ;  and  I  think  that  all  that  can  be  safely 
said,  at  present,  is  that  the  existence  of  intermediate 
forms  is  not  proven.  But,  whatever  the  constancy  of  the 


296      DISTRIBUTION  AND  AETIOLOGY  OF  THE  CRAYFISHES. 

differences  between  the  two  kinds  of  crayfishes,  there  can 
surely  be  no  doubt  as  to  their  insignificance ;  and  no 
question  that  they  are  no  more  than  such  as,  judging  by 
analog}7,  might  be  produced  \)j  variation. 

From  a  morphological  point  of  view,  then,  it  is  really 
impossible  to  decide  the  question  whether  the  stone  cray- 
fish and  the  noble  crayfish  should  be  regarded  as  species 
or  as  varieties.  But,  since  it  will,  hereafter,  be  convenient 
to  have  distinct  names  for  the  two  kinds,  I  shall  speak 
of  them  as  Astacus  torrentium  and  Astacus  nobilis.* 

In  the  physiological  sense,  a  species  means,  firstly,  a 
group  of  animals  the  members  of  which  are  capable  of 
completely  fertile  union  with  one  another,  but  not  with 
the  members  of  any  other  group  ;  and,  secondly,  it 
means  all  the  descendants  of  a  primitive  ancestor  or 
ancestors,  supposed  to  have  originated  otherwise  than  by 
ordinary  generation. 

It  is  clear  that,  even  if  crayfishes  had  an  unbegotten 
ancestor,  there  is  no  means  of  knowing  whether  the 
stone  crayfish  and  the  noble  crayfish  are  descendants  of 
the  same,  or  of  different  ancestors,  so  that  the  second 
.sense  of  species  hardly  concerns  us.  As  to  the  first 
sense,  there  is  no  evidence  to  show  whether  the  two 

*  According  to  strict  zoological  usage  the  names  should  be  written 
A,  fluviatilis  (var.  torrentium)  and  A.  fluviatilis  (var.  nobilis)  on  the 
hypothesis  that  the  stone  crayfish  and  the  noble  crayfish  are  varieties  ; 
and  A.  torrentium  and  A.  fluviatilis  on  the  hypothesis  that  they  are 
species  ;  but  as  I  neither  wish  to  prejudge  the  species  question,  nor  to 
employ  cumbrously  long  names,  I  take  a  third  course. 


PHYSIOLOGICAL   SPECIES.  297 

kinds  of  crayfish  under  consideration  are  capable  cf  fertile 
union  or  whether  they  are  sterile.  It  is  said,  however, 
that  hybrids  or  mongrels  are  not  met  with  in  the  waters 
which  are  inhabited  by  both  kinds,  and  that  the  breeding 
season  of  the  stone  crayfish  begins  earlier  than  that  of 
the  noble  crayfish. 

M.  Carbonnier,  who  practises  crayfish  culture  on  a  large 
scale,  gives  some  interesting  facts  bearing  on  this  ques- 
tion in  the  work  already  cited.  He  says  that,  in  the 
streams  of  France,  there  are  two  very  distinct  kinds  of 
crayfishes — the  red-clawed  crayfish  (L'Ecrevisse  a  pieds 
rouges),  and  the  white-clawed  crayfish  (L'Ecrevisse  a 
pieds  blancs),  and  that  the  latter  inhabit  the  swifter 
streams.  In  a  piece  of  land  converted  into  a  crayfish 
farm,  in  which  the  white-clawed  crayfish  existed  natur- 
ally in  great  abundance,  800,000  red-clawed  crayfish 
were  introduced  in  the  course  of  five  years ;  neverthe- 
less, at  the  end  of  this  time,  no  intermediate  forms  were 
to  be  seen,  and  the  "pieds  rouges"  exhibited  a  marked 
superiority  in  size  over  the  "pieds  blancs."  M.  Car- 
bonnier,  in  fact,  says  that  they  were  nearly  twice  as  big. 

On  the  whole,  the  facts  as  at  present  known,  seem  to 
incline  rather  in  favour  of  the  conclusion  that  A.  torren- 
tiam  and  A.  nobilis  are  distinct  species;  in  the  sense 
that  transitional  forms  have  not  been  clearly  made  out, 
and  that,  possibly,  they  do  not  interbreed. 

As   I   have    already  remarked,    the    very    numerous 


298      DISTRIBUTION  AND  .ETIOLOGY  OF  THE  CRAYFISHES 

specimens  of  English  and  Irish  crayfishes  which  have 
passed  through  my  hands,  have  all  presented  the  charac- 
ter of  Astacus  torrentium,  with  which  also  the  description 
given  in  works  of  recognised  authority  coincides  as  far  as 
it  goes.*  The  same  form  is  found  in  many  parts  of 
France,  as  far  south  as  the  Pyrenees,  and  it  is  met  with 
as  far  east  as  Alsace  and  Switzerland.  I  have  recently  f 
heen  enabled,  by  the  kindess  of  Dr.  Bolivar,  of  Madrid, 
who  sent  me  a  number  of  crayfishes  from  the  neighbour- 
hood of  that  city,  to  satisfy  myself  that  the  Spanish 
peninsula  contains  crayfishes  altogether  similar  to  those 
of  Britain,  except  that  the  subrostral  spine  is  less  de- 
veloped. Further,  I  have  no  doubt  that  Dr.  Heller  j  is 
right  in  his  identification  of  the  English  crayfish  with 
a  form  which  he  describes  under  the  name  of  A. 
saxatilis.  He  says  that  it  is  especially  abundant  in 
Southern  Europe,  and  that  it  occurs  in  Greece,  in 
Dalmatia,  in  the  islands  of  Cherso  and  Veglia,  at  Trieste, 
in  the  Lago  di  Garda,  and  at  Genoa.  Further,  Astacus 
torrentium  appears  to  be  widely  distributed  in  North 
Germany.  The  eastern  limit  of  thig  crayfish  is  uncertain ; 
but,  according  to  Kessler,§  it  does  not  occur  within  the 
limits  of  the  Russian  empire. 

*  See  Bell.    "  British  Stalk-eyed  Crustacea,"  p.  237. 

f  Since  the  statement  respecting  the  occurrence  of  crayfishes  in  Spain 
jn  p.  44  was  printed. 

J  "  Die  Crustaceen  des  Siidlichen  Europas,"  1863. 

§  "  Die  Russischen  Flusskrebse."  Bulletin  de  la  Societ6  Imperials 
flea  Naturalistes  de  Moscow,  1874. 


ASTACUS  NOBILIS.  299 

Astacus  torrentium  appears  to  be  particularly  addicted 
to  rapid  highland  streams  and  the  turbid  pools  which 
they  feed. 

Astacus  nobilis  is  indigenous  to  France,  Germany,  and 
the  Italian  peninsula.  It  is  said  to  be  found  at  Nice 
and  at  Barcelona,  though  I  cannot  hear  of  it  elsewhere 
in  Spain.  Its  south-eastern  limit  appears  to  be  the  Lake 
of  Zirknitz,  in  Carniola,  not  far  from  the  famous  caves  of 
Adelsberg.  It  is  not  known  in  Dalmatia,  in  Turkey,  nor 
in  Greece.  In  the  Russian  empire,  according  to  Kessler, 
this  crayfish  chiefly  inhabits  the  watershed -of  the  Baltic. 
The  northern  limit  of  its  distribution  lies  between  Chris- 
tianstad,  in  the  Gulf  of  Bothnia  (52°  16'  N),  and  Serdobol, 
at  the  northern  end  of  Lake  Ladoga.  "  Eastward  of 
Lake  Ladoga  it  is  found  in  the  Uslanka,  a  tributary  of 
the  Swir.  It  appears  to  be  the  only  crayfish  which  exists 
in  the  waters  which  flow  from  the  south  into  the  Gulf  of 
Finland  and  into  the  Baltic;  except  in  those  streams  and 
lakes  which  have  been  artificial!}'  connected  with  the  Volga, 
and  in  which  it  is  partially  replaced  by  A.  leptodactylus". 
It  still  inhabits  the  Lakes  of  Beresai  and  Bologoe,  as 
well  as  the  affluents  of  the  Msta  and  the  Wolchow ;  and 
it  is  met  with  in  affluents  of  the  Dnieper,  as  far  as 
Mohilew.  Astacus  nobilis  is  also  found  in  Denmark  and 
Southern  Sweden ;  but,  in  the  latter  country,  its  intro- 
duction appears  to  have  been  artificial.  This  crayfish 
is  said  occasionally  to  be  met  with  on  the  Livonian  coast 
in  the  waters  of  the  Baltic,  which,  however,  it  must 


MO      DISTRIBUTION   AND  .ETIOLOGY  OF  THE  CRAYFISHES. 

be  remembered,  are  much  less  salt  than  ordinary  sea 
water. 

It  will  he  observed  that  while  the  two  forms,  A.  torren- 
tium  and  A.  nobilis,  are  intermixed  over  a  large  part  of 
Central  Europe,  A.  torrentium  has  a  wider  north-west- 
ward, south-westward,  and  south-eastward  extension, 
being  the  sole  occupant  of  Britain,  and  apparently  oi 
the  greater  part  of  Spain  and  of  Greece.  On  the  other 
hand,  in  the  northern  and  eastern  parts  of  Central 
Europe,  A.  nobilis  appears  to  exist  alone. 

Further  to  the  east,  a  new  form,  Astacus  leptodactylus 
(fig.  75),  makes  its  appearance.  Whether  A.  leptodactylus 
exists  in  the  upper  waters  of  the  Danube,  does  not  appear, 
but  in  the  lower  Danube  and  in  the  Theiss  it  is  the  domi- 
nant, if  not  the  exclusive,  crayfish.  From  hence  it  extends 
through  all  the  rivers  which  flow  into  the  Black,  Azov, 
and  Caspian  Seas,  from  Bessarabia  and  Podolia  on  the 
west,  to  the  Ural  mountains  on  the  east.  In  fact,  the 
natural  habitat  of  this  crayfish  appears  to  be  the  water- 
shed of  the  Pontocaspian  area,  excluding  that  part  of  the 
Black  Sea  which  lies  southward  of  the  Caucasus  on  the 
one  hand,  and  of  the  mouths  of  the  Danube  on  the  other.* 

It  is  a  remarkable  circumstance  that  this  crayfish  not 
only  thrives  in  the  brackish  waters  of  the  estuaries  of 
the  rivers  which  debouche  into  the  Black  Sea  and  the 
Sea  of  Azov,  but  that  it  is  found  even  in  the  salter 

*  These  statements  rest  on  the  authority  of  Kessler  and  Gersfcfeldt, 
in  their  memoirs  already  cited. 


FIG.  75.~A*tacHg  leptodactylu*  (after  Rathke,  J  nat.  size). 


302      DISTEIBUTION  AND   .ETIOLOGY  OF  THE   CRAYFISHES, 

southern  parts  of  the  Caspian,  in  which  it  lives  at 
considerable  depths. 

In  the  north,  Astacus  leptodactylus  is  met  with  in  the 
rivers  which  flow  into  the  White  Sea,  as  well  as  in  many 
streams  and  lakes  about  the  Gulf  of  Finland.  B?it  it 
has  probably  been  introduced  into  these  streams  by  the 
canals  which  have  been  constructed  to  connect  the  basin 
of  the  Volga  with  the  rivers  which  flow  into  the  Baltic 
and  into  the  White  Sea.  In  the  latter,  the  invading  A. 
leptodactylus  is  everywhere  overcoming  and  driving  out 
A.  nobilis  in  the  struggle  for  existence,  apparently  in 
virtue  of  its  more  rapid  multiplication.* 

In  the  Caspian  and  in  the  brackish  waters  of  the 
estuaries  of  the  Dniester  and  the  Bug,  a  somewhat 
different  crayfish,  which  has  been  called  Astacus  pachypus, 
occurs  ;  another  closely  allied  form  ( A .  angulosus)  is  met 
with  in  the  mountain  streams  of  the  Crimea  and  of  the 
northern  face  of  the  Caucasus ;  and  a  third,  A.  colchicus, 
has  recently  been  discovered  in  the  Rion,  or  Phasis  of 
the  ancients,  which  flows  into  the  eastern  extremity  of 
the  Black  Sea. 

With  respect  to  the  question  whether  these  Ponto- 
caspian  crayfishes  are  specifically  distinct  from  one 
another,  and  whether  the  most  widely  distributed  kind, 
A.  leptodactylus,  is  distinct  from  A.  nobilis,  exactly  the 
game  difficulties  arise  as  in  the  case  of  the  west  European 

*  Kessler  (Die  Russischen  Flusskrebse,  1.  c.  p.  369-70),  has  an  in 
teresting  discussion  of  this  question. 


ASTACUS  LEPTODACTYLUS.  303 

crayfishes.  Gerstfeldt,  who  has  had  the  opportunity  of 
examining  large  series  of  specimens,  concludes  that  the 
Pontocaspian  crayfishes  and  A.  nobilis  are  all  varieties 
of  one  species.  Kessler,  on  the  contrary,  while  he 
admits  that  A.  angulosus  is,  and  A.  pachypus  may  be, 
a  variety  of  A.  leptodactylus,  affirms  that  the  latter  is 
specifically  distinct  from  A.  nobilis. 

Undoubtedly,  well  marked  examples  of  A.  leptodactylus 
are  very  different  from  A.  nobilis. 

1.  The  edges  of  the  rostrum  are  produced  into  five  or 
six  sharp  spines,  instead  of  being  smooth  or  slightly 
serrated  as  in  A.  nobilis. 

2.  The  fore   part  of  the    rostrum   has   no    serrated 
spinous  median  keel,  such  as  commonly,  though  not  uni- 
versally, exists  in  A.  nobilis. 

3.  The  posterior  end  of  the  post-orbital  ridge  is  still 
more  distinct  and  spiniform  than  in  A.  nobilis. 

4.  The  abdominal  pleura  of  A.  leptodactylus  are  nar- 
rower, more  equal  sided,  and  triangular  in  shape. 

5.  The  chelae  of  the  forceps,  especially  in  the  males, 
are  more  elongated ;    and  the  moveable  and  fixed  claws 
are  slenderer  and  have  their  opposed  edges  straighter 
and  less  tuberculated. 

But,  in  all  these  respects,  individual  specimens  of 
A.  nobilis  vary  in  the  direction  of  A.  leptodactylus  and 
vice  versd;  and  if  A.  angulosus  and  A.  pachypus  are 
varieties  of  A.  leptodactylus,  I  cannot  see  why  Gerst- 
feldt's  conclusion  that  A.  nobilis  is  another  variety  of 


304      DISTRIBUTION   AND  AETIOLOGY  OF  THE  CRAYFISHES. 

the  same  form  need  be  questioned  on  morphological 
grounds.  However,  Kessler  asserts  that,  in  those  lo- 
calities in  which  A.  leptodactylus  and  A.  nobilis  live 
together,  no  intermediate  forms  occur,  which  is  pre- 
sumptive evidence  that  they  do  not  intermix  by  breeding. 

No  crayfishes  are  known  to  inhabit  the  rivers  of  the 
northern  Asiatic  watershed,  such  as  the  Obi,  Yenisei, 
and  Lena.  None  are  known  *  in  the  sea  of  Aral,  or  the 
great  rivers  Oxus  and  Jaxartes,  which  feed  that  vast 
lake ;  nor  any  in  the  lakes  of  Balkash  and  Baikal.  If 
further  exploration  verifies  this  negative  fact,  it  will  be 
not  a  little  remarkable  ;  inasmuch  as  two  t,  if  not  more, 
kinds  of  crayfishes  are  found  in  the  basin  of  the  great 
river  Amur,  which  drains  a  large  area  of  north-eastern 
Asia,  and  debouches  into  the  Gulf  of  Tartary,  in  about 
the  latitude  of  York. 

Japan  has  one  species  (A.  japonicus),  perhaps  more ; 
but  no  crayfish  has  as  yet  been  made  known  in  any  part 
of  eastern  Asia,  south  of  Amurland.  There  are  cer- 
tainly none  in  Hindostan ;  none  are  known  in  Persia, 
Arabia,  or  Syria.  In  Asia  Minor  the  only  recorded 
locality  is  the  Rion.  No  crayfish  has  yet  been  disco- 
vered  in  the  whole  continent  of  Africa.  1 

*  It  would  be  hazardous,  however,  to  assume  that  none  exist,  especi' 
ally  in  the  Oxus,  which  formerly  flowed  into  the  Caspian. 

f  A.  dauricus  and  A.  Schrenckii. 

t  Whatever  the  so-called  Astacus  capensis  of  the  Cape  Colony  maj 
»e,  it  is  certainly  not  a  crayfish. 


NORTH   AMERICAN   CRAYFISHES.  305 

Thus,  on  the  continent  of  the  old  world,  the  crayfishes 
are  restricted  to  a  zone,  the  southern  limit  of  which 
coincides  with  certain  great  geographical  Matures ;  on 
the  west,  the  Mediterranean,  with  its  continuation,  the 
Black  Sea ;  then  the  range  of  the  Caucasus,  followed  by 
the  great  Asiatic  highlands,  as  far  as  the  Corea  on  the 
east.  On  the  north,  though  there  is  no  such  physical 
boundary,  the  crayfishes  appear  to  be  entirely  excluded 
from  the  Sibeiian  river  basins ;  while  east  and  west, 
though  a  sea-barrier  exists,  the  crayfishes  extend  beyond 
it,  to  reach  the  British  islands  and  those  of  Japan. 

Crossing  the  Pacific,  we  meet  with  some  half-a-dozen 
kinds  of  crayfishes,*  different  from  those  of  the  old 
world,  but  still  belonging  to  the  genus  Astacus,  in 
British  Columbia,  Oregon,  and  California.  Beyond  the 
Rocky  Mountains,  from  the  Great  Lakes  to  Guatemala, 
crayfishes  abound,  as  many  as  thirty-two  different  species 
having  been  described,  but  they  all  belong  to  the  genus 
Cambams  (fig.  63,  p.  248).  Species  of  this  genus  also 
occur  in  Cuba,t  but,  so  far  as  is  at  present  known,  not 
in  any  of  the  other  West  Indian  islands.  The  occurrence 
of  a  curious  dimorphism  among  the  male  Cambari  has 
been  described  by  Dr.  Hagen;  and  a  blind  Cambarus 


*  Dr.  Hagen  in  his  "  Monograph  of  the  North  American  Astacidas," 
enumerates  six  species  ;  A.  Gambelil,  A.  klamatJiensis,  A.  leenisculug, 
A.  nigresccns,  A.  oreganus,  and  A.  Trombridgii. 

f  Von  Martens.     Cambarus  eubentis.    Axchiv.  f  or  Naturgeschichte, 

21 


306       DISTRIBUTION  AND   .ETIOLOGY  OF  THE  CRAYFISHES. 

is  found,  along  with  other  blind  animals,  in  the  sub- 
terranean caves  of  Kentucky. 

All  the  crayfishes  of  the  northern  hemisphere  belong 
to  the  Potamobiida,  and  no  members  of  this  family  are 
known  to  exist  south  of  the  equator.  The  crayfishes  of  the 
southern  hemisphere,  in  fact,  all  belong  to  the  division  of 
the  Parastacidce,  and  in  respect  of  the  number  and  variety 
of  forms  and  the  size  which  they  reach,  the  head-quarters 
of  the  Parastacidce  is  the  continent  of  Australia.  Some 
of  the  Australian  crayfishes  (fig.  76)  attain  a  foot  or 
more  in  length,  and  are  as  large  as  full-sized  lobsters. 
The  genus  Engam  of  Tasmania  comprises  small  cray- 
fish which,  like  some  of  the  Cambari,  live  habitually  on 
land,  in  burrows  which  they  excavate  in  the  soil. 

New  Zealand  has  a  peculiar  genus  of  crayfishes, 
Paranephrops,  a  species  of  which  is  found  in  the  Fiji 
Islands,  but  none  are  known  to  occur  elsewhere  in 
Polynesia. 

Two  kinds  of  crayfish  have  been  obtained  in  southern 
Brazil,  and  have  been  described  by  Dr.  v.  Martens,*  as 
A.  pilimanus  and  A.  brasiliensis.  I  have  shown  that 
they  belong  to  a  peculiar  genus,  Parastacus.  The  former 
was  procured  at  Porto  Alegre,  which  is  situated  in  30° 
S.  Latitude,  close  to  the  mouth  of  the  Jacuhy,  at  the 
north  end  of  the  great  Laguna  do  Patos,  which  communi- 

*  Siidbrasilische  Suss-  und  Brackwasser  Crustaceen,  nach  den  Samm« 
lungen  des  Dr.  Keinh.  Hensel.  Archiv.  fiir  Naturgeschichte,  xxxv, 
1869. 


FIG.  76.— Australian  Crayfish  (^  nat.  size).* 

*  The  nomenclature  of  the  Australian  crayfishes  requires  thorough 
revision.     I  therefore,  for  the  present,  assign  no  name  to  this  cray- 


308      DISTRIBUTION  AND   .ETIOLOGY  OF  THE   CRAYFISHES. 

cates  by  a  narrow  passage  with  the  sea ;  and  also  at  Sta. 
Cruz  in  the  upper  basin  of  the  Eio  Pardo,  an  affluent  of 
the  Jacuhy,  "  by  digging  it  out  of  holes  in  the  ground." 
The  latter  (P.  brasiliensis,  fig.  64)  was  obtained  at  Porto 
Alegre,  and  further  inland,  in  the  region  of  the  primitive 
forest  at  Rodersburg,  in  shallow  streams. 

In  addition  to  these,  no  crayfish  have  as  yet  been 
found  in  any  of  the  great  rivers,  such  as  the  Orinoko ; 
the  Amazon,  in  which  they  were  specially  sought  foi  by 
Agassiz ;  or  in  the  La  Plata,  on  the  eastern  side  of  the 
Andes.  But,  on  the  west,  an  "Astacus"  chilensis  is 
described  in  the  "Histoire  Naturelle  des  Crustacees," 
(vol.  ii.  p.  833).  It  is  here  stated  that  this  crayfish 
"habite  les  cotes  du  Chili,"  but  the  freshwaters  of  the 
Chilian  coast  are  doubtless  to  be  understood. 

Finally,  Madagascar  has  a  genus  and  species  of  cray- 
fish (Astacoides  madagascariensis,  fig.  65)  peculiar  to  itself. 

On  comparing  the  results  obtained  by  the  study  of  the 
geographical  distribution  of  the  crayfishes  with  those 
brought  to  light  by  the  examination  of  their  morphological 
characters,  the  important  fact  that  there  is  a  broad  and 
general  correspondence  between  the  two  becomes  ap- 
parent. The  wide  equatorial  belt  of  the  earth's  surface 
which  separates  the  crayfishes  of  the  northern  from  those 
of  the  southern  hemisphere,  is  a  sort  of  geographical 

fish.  It  is  probably  identical  with  the  A.  nolilis  of  Dana  and  the  A.  ar- 
matm  of  Yon  Martens. 


W! 


If  a 


"5  « 


olO      DISTRIBUTION  AND  .ETIOLOGY   OF  THE  CRAYFISHES 

representation  of  the  broad  morphological  differences 
which  mark  off  the  Potamobiida  from  the  Parastacida. 
Each  group  occupies  a  definite  area  of  the  earth's  surface, 
and  the  two  are  separated  by  an  extensive  border-land 
tmtenanted  by  crayfishes. 

A  similar  correspondence  is  exhibited,  though  less 
distinctly,  when  we  consider  the  distribution  of  the 
genera  and  species  of  each  group.  Thus,  among  the 
Potamobiidce,  Astacus  torrentium  and  nobilis  belong  essen- 
tially to  the  northern,  western,  and  southern  watersheds 
of  the  central  European  highlands,  the  streams  of  which 
flow  respectively  into  the  Baltic  and  the  North  Seas,  the 
Atlantic  and  the  Mediterranean  (fig.  77,  I.) ;  A.  leptodac- 
tylus,  pachypus,  angulosus,  and  colchicus,  appertain  to  the 
Pontocaspian  watershed,  the  rivers  of  which  drain  into 
the  Black  Sea  and  the  Caspian  (I.)  ;  while  Astacus 
dauricus  and  A.  Schrenckii  are  restricted  to  the  widely 
separated  basin  of  the  Amur,  which  sheds  its  waters 
into  the  Pacific  (II.).  The  Astaci  of  the  rivers  of 
western  North  America,  which  flow  into  the  Pacific  (IV.), 
and  the  Cambari  of  the  Eastern  or  Atlantic  water-shed  (V.) 
are  separated  by  the  great  physical  barrier  of  the  Rocky 
Mountain  ranges.  Finally,  with  regard  to  the  Par- 
<i4acid<z,  the  widely  separated  geographical  regions  of 
New  Zealand  (VIII.),  Australia  (IX.),  Madagascar  (XII.), 
and  South  America  (VI.  and  VII.),  are  inhabited  by 
genetically  distinct  groups. 

But  when  we  look  more  closely  into  the  matter,  it  will 


MORPHOLOGICAL  AND  GEOGRAPHICAL  GROUPb.   311 


be  found  that  the  parallel  between  the  geographical  and  the 
morphological  facts  cannot  be  quite  strictly  carried  out. 

Astacus  torrentium,  as  we  have  seen,  inhabits  both 
the  British  Islands  and  the  continent  of  Europe;  never- 
theless, there  is  every  reason  to  believe  that  twenty 
miles  of  sea  water  is  an  insuperable  barrier  to  the 
passage  of  crayfishes  from  one  land  to  the  other.  For 
though  some  crayfishes  live  in  brackish  water,  there  is 
no  evidence  that  any  existing  species  can  maintain  them- 
selves in  the  sea.  A  fact  of  the  same  character  meets  us 
at  the  other  side  of  the  Eurasiatic  continent,  the  Japanese 
and  the  Amurland  crayfishes  being  closely  allied ;  although 
it  is  not  clear  that  there  are  any  identical  species  on  the 
two  sides  of  the  Sea  of  Japan. 

Another  circumstance  is  still  more  remarkable.  The 
West  American  crayfishes  are  but  little  more  different  from 
the  Pontocaspian  crayfishes,  than  these  are  from  Astacus 
torrentium.  On  the  face  of  the  matter,  one  might  there- 
fore expect  the  Amurland  and  Japanese  crayfishes,  which 
are  intermediate  in  geographical  position,  to  be  also 
intermediate,  morphologically,  between  the  Pontocaspian 
and  the  West  American  forms.  But  this  is  not  the 
case.  The  branchial  system  of  the  Amurland  Astaci 
appears  to  be  the  same  as  that  of  the  rest  of  the  genus  ; 
but,  in  the  males,  the  third  joint  (ischiopodite)  of  the 
second  and  third  pair  of  ambulatory  limbs  is  provided 
with  a  conical,  recurved,  hook-like  process ;  while,  in  the 
females,  the  hinder  edge  of  the  penultimate  thoracic 


312       DISTRIBUTION   AND   AETIOLOGY   OF   THE   CRAYFISHES. 

sternum  is  elevated  into  a  transverse  prominence,  on  the 
posterior  face  of  which  there  is  a  pit  or  depression.* 

In  both  these  characters,  but  more  especially  in  the 
former,  the  Amurland  and  Japanese  Astaci  depart  from 
both  the  Pontocaspian  and  the  West  American  Astaci, 
and  approach  the  Cambari  of  Eastern  North  America. 

In  these  crayfishes,  in  fact,  one  or  both  of  the  same 
pairs  of  lego  in  the  male  are  provided  with  similar 


Fra.  78. — Cambarus  (Guatemala)  penultimate  leg.  cap,  coxopodite  ; 
cx.s,  coxopoditic  setas  ;  pdb,  podobranchia ;  bp,  basipodite  ;  ip,  ischiopo 
dite ;  mpt  meropodite  ;  cp  carpopodite ;  pp,  propodite ;  dp,  dactylopodite. 


hook-like  processes ;  while,  in  the  females,  the  modifi- 
cation of  the  penultimate  thoracic  sternum  is  carried 
still  further  and  gives  rise  to  the  curious  structure  de- 
scribed by  Dr.  Hagen  as  the  "  annulus  ventralis." 

In  all  the  Cambari,  the  pleurobraiichise  appear  to  be 
entirely  suppressed,  and  the  hindermost  podobranchia  has 
no  lamina  ;  while  the  areola  is  usually  extremely  narrow. 
The  proportional  size  of  the  areola  in  the  Amurland 

*  Kessler,  1.  c. 


MORPHOLOGICAL  AND  GEOGRAPHICAL  GROUPS.   313 

crayfishes  is  not  recorded;  in  the  Japanese  crayfish, 
judging  by  the  figure  given  hy  De  Haan,  it  is  about  the 
same  as  in  the  western  Astaci.  On  the  other  hand,  in 
the  West  American  crayfishes  it  is  distinctly  smaller ;  so 
that,  in  this  respect,  they  perhaps  more  nearly  approach 
the  Cambari.  Unfortunately,  nothing  is  known  as  to 
the  branchiae  of  the  Amurland  crayfishes.  According 
to  De  Haan,  those  of  the  Japanese  species  resemble 
those  of  the  western  Astaci:  as  those  of  the  West 
American  Astaci  certainly  do. 

With  respect  to  the  Parastacida ;  in  the  remarkable 
length  and  flatness  of  the  epistoma,  the  crayfishes  of 
Australia,  Madagascar,  and  South  America,  resemble 
one  another.  But  in  its  peculiar  truncated  rostrum  (see 
fig.  65)  and  in  the  extreme  modification  of  its  branchial 
system,  which  I  have  described  elsewhere,  the  Madagascar 
genus  stands  alone. 

The  Paranephrops  of  New  Zealand  and  the  Fijis,  with 
its  wide  and  short  epistoma,  long  rostrum,  and  large 
antennary  squaines,  is  much  more  unlike  the  Australian 
forms  than  might  be  expected  from  its  geographical 
position.  On  the  other  hand,  considering  their  wide 
separation  by  sea,  the  amount  of  resemblance  be- 
tween the  New  Zealand  and  the  Fiji  species  is  very 
remarkable. 

If  the  distribution  of  the  crayfishes  is  compared 
with  that  of  terrestrial  animals  in  general,  the  points  of 


314      DISTRIBUTION   AND   AETIOLOGY   OF   THE   CRAYFISHES. 

difference    are    at    least    as    remarkable   as   the    resem- 
blances. 

With  respect  to  the  latter,  the  area  occupied  by  the 
Potamobiidce,  corresponds  roughly  with  the  Palaearctic 
and  Nearctic  divisions  of  the  great  Arctogaal  provinces 
of  distribution  indicated  by  mammals  and  birds;  while 
distinct  groups  of  crayfishes  occupy  a  larger  or  smaller 
part  of  the  other,  namely,  the  Austro- Columbian,  Aus- 
tralian, and  Novozelanian  primary  distributional  pro- 
vinces of  mammals  and  birds.  Again,  the  peculiar 
crayfishes  of  Madagascar  answer  to  the  special  features 
of  the  rest  of  the  fauna  of  that  island. 

But  the  North  American  crayfishes  extend  much 
further  South  than  the  limits  of  the  Nearctic  fauna  in 
general ;  while  the  absence  of  any  group  of  crayfishes 
in  Africa,  or  in  the  rest  of  the  old  world,  south  of  the 
great  Asiatic  table-land,  forms  a  strong  contrast  to  the 
general  resemblance  of  the  North  African  and  Indian 
fauna  to  that  of  the  rest  of  Arctogaea.  Again,  there  is 
no  such  vast  difference  between  the  crayfishes  of  New 
Zealand,  Australia,  and  South  America,  as  there  is 
between  the  mammals  and  the  birds  of  those  regions. 

It  may  be  concluded,  therefore,  that  the  conditions 
which  have  determined  the  distribution  of  crayfishes  have 
been  very  different  from  those  which  have  governed  the 
distribution  of  mammals  and  birds.  But  if  we  compare 
with  the  distribution  of  the  crayfishes,  not  that  of  ter- 
restrial animals  in  general,  but  only  that  of  freshwater 


THE  DISTRIBUTION   OF  FRESHWATER  CRAYFISHES.      315 

fishes,  some  very  curious  points  of  approximation  become 
manifest.  The  Salmonidce,  or  fishes  of  the  salmon  and 
trout  kind,  a  few  of  which  are  exclusively  marine,  many 
both  marine  and  freshwater,  while  others  are  confined 
to  fresh  water,  are  distributed  over  the  northern  hemi- 
sphere, in  a  manner  which  recalls  the  distribution  of 
the  Potamobine  crayfishes,*  though  they  do  not  extend 
so  far  to  the  Scuth  in  the  new  world,  while  they  go  a 
little  further,  namely,  as  far  as  Algeria,  Northern  Asia 
Minor,  and  Armenia,  in  the  old  world.  With  the  excep- 
tion of  the  single  genus  Retropinna,  which  inhabits  New 
Zealand,  no  true  salmonoid  fish  occurs  south  of  the 
equator  ;  but,  as  Dr.  Giinther  has  pointed  out,  two 
groups  of  freshwater  fishes,  the  Haplochitonida  and  the 
Galaxidce,  which  stand  in  somewhat  the  same  relation  to 
the  Sahnonidce  as  the  Parastacida  do  to  the  Pohimobiida, 
take  the  place  of  the  Salmonidce  in  the  fresh  waters  of 
New  Zealand,  Australia,  and  South  America.  There 
are  two  species  of  Haplochiton  in  Tierra  del  Fuego  ;  and 
of  the  closely  allied  genus  Prototrociex,  one  species  is 
found  in  South  Australia,  and  one  in  New  Zealand  ;  of 
the  Galaxidce,  the  same  species,  Galaxias  attennuatus, 
occurs  in  the  streams  of  New  Zealand,  Tasmania,  the 
Falkland  Islands,  and  Peru. 

Thus,  these  fish  avoid  South  Africa,  as  the  crayfishes 

*  According  to  Dr.  Giinther  their  soutiiern  range  is  similarly  limited  by 
the  Asiatic  Highlands.  But  they  abound  in  the  rivers  both  of  the  ola 
And  new  worlds  which  flow  into  the  Arctic  sea  ;  and  though  those  on 


316      DISTRIBUTION  AND   .ETIOLOGY  OF  THE  CRAYFISHES. 

do  ;  but  I  am  not  aware  that  any  member  of  the  group  is 
found  in  Madagascar,  and  thus  completes  the  analogy. 

The  preservation  of  the  soft  parts  of  animals  in  the 
fossil  state  depends  upon  favourable  conditions  of  rare 
occurrence  ;  and,  in  the  case  of  the  Crustacea,  it  is  not 
often  that  one  can  hope  to  meet  with  such  small  hard 
parts  as  the  abdominal  members,  in  a  good  state  of 
preservation.  But  without  recourse  to  the  branchial 
apparatus,  and  to  the  abdominal  appendages,  it  might  be 
very  difficult  to  say  whether  a  given  crustacean  belonged 
to  the  Astacine,  or  to  the  closely  allied  Homarine  group. 
Of  course,  if  the  accompanying  fossils  indicated  that  the 
deposit  in  which  the  remains  occur,  was  of  freshwater 
origin,  the  presumption  in  favour  of  their  Astacine  nature 
would  be  very  strong  ;  but  if  they  were  inhabitants  of  the 
sea,  the  problem  whether  the  crustacean  in  question  was 
a  marine  Astacine,  or  a  true  Homarine,  might  be  very 
hard  to  solve. 

Undoubted  remains  of  crayfishes  have  hitherto  been 
discovered  only  in  freshwater  strata  of  late  tertiary  age. 
In  Idaho,  North  America,  Professor  Cope  *  found,  in 
association  with  Mastodon  mirificus,  and  Equus  excelsus, 
several  species,  which  he  considers  to  be  distinct  from 

the  western  side  of  the  Rocky  Mountains  are  different  from  the  Eastern 
American  forms,  yet  there  are  species  common  to  both  the  Asiatic  and 
the  American  coasts  of  the  North  Pacific. 

*  On  three  extinct  Astaci  from  the  freshwater  Tertiary  of  Idaho.  Pro- 
ceedings of  the  American  Philosophical  Society,  1869-70. 


THE   ETIOLOGY  OF  THE  CRAYFISHES.  317 

the  existing  American  crayfishes;  whether  they  are 
Camban  or  Astaci  does  not  appear.  But,  in  the  lower 
chalk  of  Ochtrup,  in  Westphalia,  and  therefore  in  a 
marine  deposit,  Von  der  Marck  and  Schliiter*  have 
obtained  a  single,  somewhat  imperfect,  specimen  of  a 
crustacean,  which  they  term  Astaciis  polit us,  and  which, 
singularly  enough,  has  the  divided  telson  found  only  in 
the  genus  Astacus.  It  would  be  very  desirable  to  know 
more  about  this  interesting  fossil.  For  the  present  it 
affords  a  strong  presumption  that  a  marine  Potamobine 
existed  as  far  back  as  the  earlier  part  of  the  cretaceous 
epoch. 

Such  are  the  more  important  facts  of  Morphology, 
Physiology,  and  Distribution,  which  make  up  the  sum 
of  our  present  knowledge  of  the  Biology  of  Crayfishes. 
The  imperfection  of  that  knowledge,  especially  as  re- 
gards the  relations  between  Morphology  and  Distribution, 
becomes  a  serious  drawback  when  we  attack  the  final 
problem  of  Biology,  which  is  to  find  out  why  animals 
of  such  structure  and  active  powers,  and  so  localized, 
exist? 

It  would  appear  difficult  to  frame  more  than  two 
fundamental  hypotheses  in  attempting  to  solve  this  pro- 
blem. Either  we  must  seek  the  origin  of  crayfishes  in 
conditions  extraneous  to  the  ordinary  course  of  natural 

*  Neue  Fische  und  Krebse  ana  der  Kreide  von  Westphalen.     Palaeon« 
tographica,  Bd.  XV.,  p.  302  ;  tab.  XLIV.,  figs.  4  and  5. 


318      DISTRIBUTION   AND   AETIOLOGY   OF  THE   CRAYFISHEa 

operations,  by  what  is  commonly  termed  Creation;  or  we 
must  seek  for  it  in  conditions  afforded  by  the  usual 
course  of  nature,  when  the  hypothesis  assumes  some 
shape  of  the  doctrine  of  Evolution.  And  there  are  two 
furms  of  the  latter  hypothesis ;  for,  it  may  be  assumed, 
on  the  one  hand,  that  crayfishes  have  come  into  exist- 
ence, independently  of  any  other  form  of  living  matter 
which  is  the  hypothesis  of  spontaneous  or  equivocal 
generation,  or  abiogenesis ;  or,  on  the  other  hand, 
we  may  suppose  that  crayfishes  have  resulted  from  the 
modification  of  some  other  form  of  living  matter ;  and 
this  is  what,  to  borrow  a  useful  word  from  the  French 
language,  is  known  as  transformism. 

I  do  not  think  that  any  hypothesis  respecting  the 
origin  of  crayfishes  can  be  suggested,  which  is  not 
referable  to  one  or  other  of  these,  or  to  a  combination 
of  them. 

As  regards  the  hypothesis  of  creation,  little  need  be 
said.  From  a  scientific  point  of  view,  the  adoption  of 
this  speculation  is  the  same  thing  as  an  admission  that 
the  problem  is  not  susceptible  of  solution.  Moreover, 
the  proposition  that  a  given  thing  has  been  created, 
whether  true  or  false,  is  not  capable  of  proof.  By 
the  nature  of  the  case  direct  evidence  of  the  fact  is 
not  obtainable.  The  only  indirect  evidence  is  such 
as  amounts  to  proof  that  natural  agencies  are  incom- 
petent to  cause  the  existence  of  the  thing  in  question. 
But  such  evidence  is  out  of  our  reach.  The  most  that 


CREATION  AND  EVOLUTION.          319 

can  be  proved,  in  any  case,  is  that  no  known  natural 
cause  is  competent  to  produce  a  given  effect ;  and  it  is 
an  obvious  blunder  to  confound  the  demonstration  of  our 
own  ignorance  with  a  proof  of  the  impotence  of  natural 
causes.  However,  apart  from  the  philosophical  worth- 
lessness  of  the  hypothesis  of  creation,  it  would  be  a  waste 
of  time  to  discuss  a  view  which  no  one  upholds.  And, 
unless  I  am  greatly  mistaken,  at  the  present  day,  no 
one  possessed  of  knowledge  sufficient  to  give  his  opinion 
importance  is  prepared  to  maintain  that  the  ancestors  of 
the  various  species  of  crayfish  were  fabricated  out  of  in- 
organic matter,  or  brought  from  nothingness  into  being, 
by  a  creative  fiat. 

Our  only  refuge,  therefore,  appears  to  be  the  hypo- 
thesis of  evolution.  And,  with  respect  to  the  doctrine 
of  abiogenesis,  we  may  also,  in  view  of  a  proper 
economy  of  labour,  postpone  its  discussion  until  such 
time  as  the  smallest  fragment  of  evidence  that  a  crayfish 
can  be  evolved  by  natural  agencies  from  not  living  matter, 
is  brought  forward. 

In  the  meanwhile,  the  hypothesis  of  transformism 
remains  in  possession  of  the  field ;  and  the  only  pro- 
fitable inquiry  is,  how  far  are  the  facts  susceptible  of 
interpretation,  on  the  hypothesis  that  all  the  existing 
kinds  of  crayfish  are  the  product  of  the  metamorphosis 
of  other  forms  of  living  beings ;  and  that  the  bio- 
logical phenomena  which  they  exhibit  are  the  results 
of  the  interaction,  through  past  time,  of  two  series  of 


320       DISTRIBUTION   AND  .ETIOLOGY   OF  THE   CRAYFISHES. 

factors :  the  one,  a  process  of  morphological  and  con- 
comitant physiological  modification  ;  the  other,  a  process 
of  change  in  the  condition  of  the  earth's  surface. 

If  we  set  aside,  as  not  worth  serious  consideration,  the 
assumption  that  the  Astacus  torrentium  of  Biitain  was 
originally  created  apart  from  the  Astacus  torrentium  of 
the  Continent ;  it  follows,  either  that  this  crayfish  has 
passed  across  the  sea  by  voluntary  or  involuntary  migra- 
tion ;  or  that  the  Astacus  torrentium  existed  before  the 
English  Channel,  and  spread  into  England  while  these 
islands  were  still  continuous  with  the  European  main- 
land ;  and  that  the  present  isolation  of  the  English  cray- 
fishes from  the  members  of  the  same  species  on  the 
Continent  is  to  be  accounted  for  by  those  changes  in  the 
physical  geography  of  western  Europe  which,  as  there  is 
abundant  evidence  to  prove,  have  separated  the  British 
Islands  from  the  mainland. 

There  is  no  evidence  that  our  crayfish  has  been 
purposely  introduced  by  human  agency  into  Great 
Britain ;  and  from  the  mode  of  life  of  crayfish  and  the 
manner  in  which  the  eggs  are  carried  about  by  the 
parent  during  their  development,  transport  by  birds  or 
floating  timber  would  seem  to  be  out  of  the  question. 
Again,  although  Astacus  nobilis  is  said  to  venture  into 
the  brackish  waters  of  the  Gulf  of  Finland,  and  A.  lepto- 
dactylus,  as  we  have  seen,  makes  itself  at  home  in  the 
more  or  less  salt  Caspian,  there  is  no  reason  to  believe 
that  Astacus  torrentium  is  capable  of  existing  in  sea- 


THE  DISTRIBUTION  OF  THE   CRAYFISHES.  321 

water,  still  less  of  crossing  the  many  miles  of  sea  which 
separate  England  from  even  the  nearest  point  of  the 
Continent.  In  fact,  the  existence  of  the  same  kind  of 
crayfish  on  both  sides  of  the  Channel  appears  to  be 
only  a  case  of  the  general  truth,  that  the  Fauna  of  the 
British  Islands  is  identical  with  a  part  of  that  of  the 
Continent ;  and  as  our  foxes,  badgers,  and  moles  cer- 
tainly have  neither  swum  across,  nor  been  transported 
by  man,  but  existed  in  Britain  while  it  was  still  con- 
tinuous with  western  Europe,  and  have  been  isolated 
by  the  subsequent  intervention  of  the  sea,  so  we  may 
confidently  explain  the  presence  of  Astacus  torrentium 
by  reference  to  the  same  operation. 

If  we  take  into  account  the  occurrence  of  Astacus 
nobilis  over  so  large  a  part  of  the  area  occupied  by 
Astacus  torrentium ;  its  absence  in  the  British  Islands, 
and  in  Greece ;  and  the  closer  affinity  which  exists  be- 
tween A.  nobilis  and  A.  leptodactylus,  than  between  A. 
nol/dis  and  A.  torrentium;  it  seems  not  improbable  that 
Astacus  torrentium  was  the  original  tenant  of  the  whole 
western  European  area  outside  the  Ponto-Caspian  water- 
shed ;  and  that  A.  nobilis  is  an  invading  offshoot  of  the 
Ponto-Caspian  or  leptodactylus  form  which  has  made  its 
way  into  the  western  rivers  in  the  course  of  the  many 
changes  of  level  which  central  Europe  has  undergone ; 
in  the  same  way  as  A.  leptodactylus  is  now  passing  into 
the  rivers  of  the  Baltic  provinces  of  Russia. 

The  study  of  the  glacial  phenomena  of  central  Europe 
22 


322      DISTRIBUTION  AND  .ETIOLOGY  OF  THE  CRAYFISHES. 

has  led  Sartorius  von  Waltershausen  *  to  the  conclusion 
that  at  the  time  when  the  glaciers  of  the  Alps  had  a 
much  greater  extension  than  at  present,  a  vast  mass  of 
freshwater  extended  from  the  valley  of  the  Danube  to 
that  of  the  Rhone,  around  the  northern  escarpment  of  the 
Alpine  chain,  and  connected  the  head- waters  of  the 
Danube  with  those  of  the  Rhine,  the  Rhone,  and  the 
northern  Italian  rivers.  As  the  Danube  debouches  into 
the  Black  Sea,  and  this  was  formerly  connected  with 
the  Aralo-Caspian  Sea,  an  easy  passage  would  thus  be 
opened  up  by  which  crayfishes  might  pass  from  the  Aralo- 
Caspian  area  to  western  Europe.  If  they  spread  by  this 
road,  the  Astacus  torrentium  may  represent  the  first  wave 
of  migration  westward,  while  A.  nobilis  answers  to  a 
second,  and  A.  leptodactylus,  with  its  varieties,  remains 
as  the  representative  of  the  old  Aralo-Caspian  crayfishes. 
And  thus  the  crayfishes  would  present  a  curious  parallel 
with  the  Iberian,  Aryan,  and  Mongoloid  streams  of  west- 
ward movement  among  mankind. 

If  we  thus  suppose  the  western  Eurasiatic  crayfishes 
to  be  simply  varieties  of  a  primitive  Aralo-Caspian  stock, 
their  limitation  to  the  south  by  the  Mediterranean  and  by 
the  great  Asiatic  highlands  becomes  easily  intelligible. 

The  extremely  severe  climatal  conditions  which  obtain 
in  northern  Siberia  may  sufficiently  account  for  the 

*  "  Untersuchungen  ueber  die  Klimate  der  Gegen  wart  und  der  Vorvrelt." 
Natuurkundige  Verhandelingen  van  de  Hollandsche  Maatschappij  dei 
Wetenschappen  te  Haarlem,  1865. 


CHANGES   IN   PHYSICAL   GEOGRAPHY.  323 

absence  of  crayfishes  (if  the}'  are  really  absent)  in  the 
rivers  Obi,  Yenisei,  and  Lena,  and  in  the  great  lake 
Baikal,  which  lies  .more  than  1,300  feet  above  the  sea, 
and  is  frozen  over  from  November  to  May.  Moreover, 
there  cun  be  no  doubt  that,  at  a  comparatively  recent 
period,  the  whole  of  this  region,  from  the  Baltic  to  the 
mouth  of  the  Lena,  was  submerged  beneath  a  southward 
extension  of  the  waters  of  the  Arctic  ocean  to  the  Aralo- 
Caspian  Sea  and  Lake  Baikal,  and  a  westward  extension 
to  the  Gulf  of  Finland. 

The  great  lakes  and  inland  seas  which  stretch,  at 
intervals,  from  Baikal,  on  the  east,  to  Wenner  in  Sweden, 
on  the  west,  are  simply  pools,  isolated  partly  by  the  rising 
of  the  ancient  sea-bottom  and  partly  by  evaporation;  and 
often  completely  converted  into  fresh  water  by  the  inflow 
of  the  surrounding  land-drainage.  But  the  population 
of  these  pools  was  originally  the  same  as  that  of  the 
Northern  Ocean,  and  a  few  species  of  marine  crustaceans, 
mollusks,  and  fish,  besides  seals,  remain  in  them  as 
living  evidences  of  the  great  change  which  has  taken  place. 
The  same  process  which,  as  we  shall  see,  has  isolated 
the  My  sis  of  the  Arctic  seas  in  the  lakes  of  Sweden  and 
Finland,  has  shut  up  with  it  other  arctic  marine  Crustacea, 
such  as  species  of  Gammarus  and  Idothea.  And  the  very 
same  species  of  Gammarus  is  imprisoned,  along  with 
arctic  seals,  in  the  waters  of  Lake  Baikal. 

The  distribution  of  the  American  crayfishes  agrees 
equally  well  with  the  hypothesis  of  the  northern  origin  of 


324      DISTRIBUTION  AND  .ETIOLOGY   OF  THE  CRAYFISHES. 

the  stock  from  which  they  have  been  tvolved.  Even 
under  existing  geographical  conditions,  an  affluent  of  the 
Mississippi,  the  St.  Peter's  river,  communicates  directly, 
in  rainy  weather,  with  the  Red  river,  which  flows  into 
Lake  Winnipeg,  the  southernmost  of  the  long  series  of 
intercommunicating  lakes  and  streams,  which  occupy  the 
low  and  flat  water-parting  between  the  southern  and  the 
northern  watersheds  of  the  North  American  Continent. 
But  the  northernmost  of  these,  the  Great  Slave  Lake, 
empties  itself  by  the  Mackenzie  river  into  the  Arctic 
Ocean,  and  thus  provides  a  route  by  which  crayfishes 
might  spread  from  the  north  over  all  parts  of  North 
America  east  of  the  Rocky  Mountains. 

The  so-called  Rocky  Mountain  range  is,  in  reality,  an 
immense  table-land,  the  edges  of  which  are  fringed  by  two 
principal  lines  of  mountainous  elevations.  The  table- 
land itself  occupies  the  place  of  a  great  north  and  south 
depression  which,  in  the  cretaceous  epoch,  was  occupied  by 
the  sea  and  probably  communicated  with  the  ocean  at  its 
northern,  as  well  as  at  its  southern  end.  During  and 
since  this  epoch  it  became  gradually  filled  up,  and  it 
now  contains  an  immense  thickness  of  deposits  of  all 
ages  from  the  cretaceous  to  the  pliocene — the  earlier 
marine,  the  later  more  and  more  completely  freshwater. 
During  the  tertiary  epoch,  various  portions  of  this  area 
have  been  occupied  by  vast  lakes,  the  more  northern  of 
which  doubtless  had  outlets  into  the  Northern  sea.  That 
crayfish  existed  in  the  vicinity  of  the  Rocky  Mountains 


CHANGES  IN  PHYSICAL  GEOGRAPHY.  325 

in  the  latter  part  of  the  tertiary  epoch  is  testified  by  the 
Idaho  fossils.  And  there  is  thus  no  difficulty  in  under- 
standing their  presence  in  the  rivers  which  have  now  cut 
their  way  to  the  Pacific  coast. 

The  similarity  of  the  crayfish  of  the  Amurland  and  of 
Japan  is  a  fact  of  the  same  order  as  the  identity  of  the 
English  crayfish  with  the  Astacus  torrentium  of  the  Euro- 
pean Continent,  and  is  to  be  explained  in  an  analogous 
fashion.  For  there  can  be  no  doubt  that  the  Asiatic 
continent  formerly  extended  much  further  to  the  east- 
ward than  it  does  at  present,  and  included  what  are  now 
the  islands  of  Japan.  Even  with  this  alteration  of  the 
geographical  conditions,  however,  it  is  not  easy  to  see 
how  crayfishes  can  have  got  into  the  Amur-Japanese 
fresh  waters.  For  a  north-eastern  prolongation  of  the 
Asiatic  highlands,  which  ends  to  the  north  in  the  Sta- 
novoi  range,  shuts  in  the  Amur  basin  on  the  west ;  while 
the  Amur  debouches  into  the  sea  of  Okhotsk,  and  the 
Pacific  ocean  washes  the  shores  of  the  Japanese  islands. 

But  there  are  many  grounds  for  the  conclusion  that,  in 
the  latter  half  of  the  tertiary  epoch,  eastern  Asia  and 
North  America  were  connected,  and  that  the  chain  of  the 
Kurile  and  Aleutian  islands  may  indicate  the  position  of 
a  great  extent  of  submerged  land.  In  that  case,  the  sea 
of  Okhotsk  and  Behring's  sea  may  occupy  the  site  of 
inland  waters  which  formerly  placed  the  mouth  of  the 
Amur  in  direct  communication  with  the  Northern  Ocean, 
just  as  the  Black  Sea,  at  present,  brings  the  basin  of  the 


326      DISTRIBUTION  AND  .ETIOLOGY  OF   THE  CRAYFISHES. 

Danube  into  connection,  first  with  the  Mediterranean  and 
then  with  the  western  Atlantic ;  and,  as  in  former  times, 
it  gave  access  from  the  south  to  the  vast  area  now 
drained  by  the  Volga.  When  the  Black  Sea  communi- 
cated with  the  Aralo-Caspian  sea,  and  this  opened  to 
che  north  into  the  Arctic  sea,  a  chain  of  great  inland 
waters  must  have  skirted  the  eastern  frontier  of  Europe, 
just  such  as  would  now  lie  on  the  eastern  frontier  of  Asia 
if  the  present  coast  underwent  elevation. 

Supposing,  however,  that  the  ancestral  forms  of  the 
Potamobiidce  obtained  access  to  the  river  basins  in  which 
they  are  now  found,  from  the  north,  the  hypothesis  that 
a  mass  of  fresh  water  once  occupied  a  great  part  of  the 
region  which  is  now  Siberia  and  the  Arctic  Ocean,  would 
be  hardly  tenable,  and  it  is,  in  fact,  wholly  unnecessary 
for  our  present  purpose. 

The  vast  majority  of  the  stalk- eyed  crustaceans  are,  and 
always  have  been,  exclusively  marine  animals ;  the  cray- 
fishes, the  Atyidce,  and  the  fluviatile  crabs  (Thtlphusida), 
being  the  only  considerable  groups  among  them  which 
habitually  confine  themselves  to  fresh  waters.  But 
even  in  such  a  genus  as  Penaus,  most  of  the  species 
of  which  are  exclusively  marine,  some,  such  as  Penaeus 
brasiliensis,  ascend  rivers  for  long  distances.  More- 
over, there  are  cases  in  which  it  cannot  be  doubted 
that  the  descendants  of  marine  Crustacea  have  gradually 
accustomed  themselves  to  fresh  water  conditions,  and 
have,  at  the  same  time,  become  more  or  less  modified, 


CONVERSION  OF  MARINE  INTO  FRESHWATER  ANIMALS.  327 

so  that  they  are  no  longer  absolutely  identical  with  those 
descendants  of  their  ancestors  which  have  continued  to 
live  in  the  sea.* 

In  several  of  the  lakes  of  Norway,  Sweden  and 
Finland,  and  in  Lake  Ladoga,  in  Northern  Europe ;  in 
Lake  Superior  and  Lake  Michigan,  in  North  America; 
a  small  crustacean,  My  sis  relicta,  occurs  in  such  abund- 
ance as  to  furnish  a  great  part  of  the  supply  of  food  to 
the  fresh  water  fishes  which  inhabit  these  lakes.  Now, 
this  Mysis  relicta  is  hardly  distinguishable  from  the 
My  sis  oculata  which  inhabits  the  Arctic  seas,  and  is 
certainly  nothing  but  a  slight  variety  of  that  species. 

In  the  case  of  the  lakes  of  Norway  and  Sweden,  there 
is  independent  evidence  that  they  formerly  communicated 
with  the  Baltic,  and  were,  in  fact,  fiords  or  arms  of  the 
sea.  The  communication  of  these  fiords  with  the  sea 
having  been  gradually  cut  off,  the  marine  animals  they 
contained  have  been  imprisoned ;  and  as  the  water  has 
been  slowly  changed  from  salt  to  fresh  by  the  drainage 
of  the  surrounding  land,  only  those  which  were  able  to 
withstand  the  altered  conditions  have  survived.  Among 
these  is  the  Mysis  oculata,  which  has  in  the  meanwhile 
undergone  the  slight  variation  which  has  converted  it 
into  Mysis  relicta.  Whether  the  same  explanation  ap- 

*  See  on  this  interesting  subject:  Martens,  "On  the  occurrence  of 
marine  animal  forms  in  fresh  water."  Annals  of  Natural  History,  1858 : 
Loven.  "  Ueber  einige  im  Wetter  und  Wener  See  gef undene  Crustaceen." 
Halle  Zeitschrift  fur  die  Gesammten  Wissenschaften,  xix.,  1862 :  G.  O, 
Sars,  "  Histoire  Naturelle  des  Crustaces  d'eau  douce  de  Norvege,"  1867. 


328      DISTRIBUTION   AND   .ETIOLOGY   OF   THE   CRAYFISHES, 

plies  to  Lakes  Superior  and  Michigan,  or  whether  the 
My  sis  oculata  has  not  passed  into  these  masses  of  fresh 
water  by  channels  of  communication  with  the  Arctic 
Ocean  which  no  longer  exist,  is  a  secondary  question. 
The  fact  remains  that  Mysis  relicta  is  a  primitively 
marine  animal  which  has  become  completely  adapted  to 
fresh- water  life. 

Several  species  of  prawns  (Palamori)  abound  in  our 
own  seas.  Other  marine  prawns  are  found  on  the  coasts 
of  North  America,  in  the  Mediterranean,  in  the  South 
Atlantic  and  Indian  Oceans,  and  in  the  Pacific  as  far 
south  as  New  Zealand.  But  species  of  the  same  genus 
(Palcemon)  are  met  with,  living  altogether  in  fresh  water, 
in  Lake  Erie,  in  the  rivers  of  Florida,  in  the  Ohio,  in 
the  rivers  of  the  Gulf  of  Mexico,  of  the  West  India 
Islands  and  of  eastern  South  America,  as  far  as  southern 
Brazil,  if  not  further ;  in  those  of  Chili  and  those  of 
Costa  Rica  in  western  South  America  ;  in  the  Upper 
Nile,  in  West  Africa,  in  Natal,  in  the  Islands  of  Johanna, 
Mauritius,  and  Bourbon,  in  the  Ganges,  in  the  Molucca 
and  Philippine  Islands,  and  probably  elsewhere. 

Many  of  these  fluviatile  prawns  differ  from  the  marine 
species  not  only  in  their  great  size  (some  attaining  a  foot 
or  more  in  length),  but  still  more  remarkably  in  the  vast 
development  of  the  fifth  pair  of  thoracic  appendages. 
These  are  always  larger  than  the  slender  fourth  pair 
(which  answer  to  the  forceps  of  the  crayfishes)  ;  and,  in 
the  males  especially,  they  are  very  long  and  strong,  and 


.FBESHWATEll    PRAWNS.  329 

are  terminated  by  great  cheLe,  not  unlike  those  of  the 
crayfishes.  Hence  these  fluviatile  prawns  (known  in 
many  places  by  the  name  of  "  Cammarons  ")  are  not 
unfrequently  confounded  with  true  crayfishes;  though 


B 


FIG.    79.      Palo&mon  jamfiifrnsits    (about  f  nat.   size).     A.  female; 
B,  fifth  thoracic  appendage  of  male. 

the  fact  that  there  are  only  three  pair  of  ordinary  legs 
behind  the  largest,  forceps-like  pair,  is  sufficient  at  once 
to  distinguish  them  from  any  of  the  Astacidce. 

Species    of    these   large  -  clawed    prawns   live   in   the 


330      DISTRIBUTION  AND  .ETIOLOGY  OF  THE   CRAYFISHES 

brackish  water  lagoons  of  the  Gulf  of  Mexico,  but  I 
am  not  aware  that  any  of  them  have  yet  been  met  with 
in  the  sea  itself.  The  Palcemon  lacustris  (Anchistia 
migratoria,  Heller)  abounds  in  fresh-water  ditches  and 
canals  between  Padua  and  Venice,  and  in  the  Lago  di 
Garda,  as  well  as  in  the  brooks  of  Dalmatia ;  but  its 
occurrence  in  the  Adriatic  or  the  Mediterranean,  which 
has  been  asserted,  appears  to  be  doubtful.  So  the  Nile 
prawn,  though  very  similar  to  some  Mediterranean 
prawns,  does  not  seem  to  be  identical  with  any  at 
present  known.* 

In  all  these  cases,  it  appears  reasonable  to  apply  the 
analogy  of  the  Mysis  relicta,  and  to  suppose  that  the 
fluviatile  prawns  are  simply  the  result  of  the  adaptive 
modification  of  species  which,  like  their  congeners,  were 
primitively  marine. 

But  if  the  existing  sea  prawns  were  to  die  out,  or  to 
be  beaten  in  the  struggle  for  existence,  we  should  have, 
scattered  over  the  world  in  isolated  river  basins,  more 
or  less  distinct  species  of  freshwater  prawns,  t  the  areas 
inhabited  by  which  might  hereafter  be  indefinitely  en- 
larged or  diminished,  by  alteration  in  the  elevation  of  the 

*  Heller,  "  Die  Crustaceen  dee  siidlichen  Europas,"  p.  259.  Klunzing-er, 
"  Ueber  eine  Siisswasser-crustacee  im  Nil,"  with  the  notes  by  von  Mar- 
tens and  von  Siebold:  Zeitschrift  fur  Wissenschaftliche  Zoologie,  I860. 

f  This  seems  actually  to  have  happened  in  the  case  of  the  widely, 
spread  allies  and  companions  of  the  fluviatile  prawns,  Atya  and  Cari- 
dina.  1  am  not  aware  that  truly  marine  species  of  these  genera  are 
known. 


THE  ORIGIN  OF  CRAYFISHES.  331 

land  and  by  other  changes  in  physical  geography.  And, 
indeed,  under  these  circumstances,  the  freshwater  prawns 
thomselves  might  become  so  much  modified,  that,  even  if 
the  descendants  of  their  ancestors  remained  unchanged 
in  structure  and  habits  in  the  sea,  the  relationship  of  the 
two  might  no  longer  be  obvious. 

These  considerations  appear  to  me  to  indicate  the  di- 
rection in  which  we  must  look  for  a  rational  explanation 
of  the  origin  of  crayfishes  and  their  present  distribution. 

I  have  no  doubt  that  they  are  derived  from  ancestors 
which  lived  altogether  in  the  sea,  as  the  great  majority  of 
the  MysidcB  and  many  of  the  prawns  do  now  ;  and  that,  of 
these  ancestral  crayfishes,  there  were  some  which,  like 
Mysis  oculata  or  Penaus  brasiliensis,re9idi\y  adapted  them- 
selves to  fresh  water  conditions,  ascended  rivers,  and  took 
possession  of  lakes.  These,  more  or  less  modified,  have 
given  rise  to  the  existing  crayfishes,  while  the  primitive 
stock  would  seem  to  have  vanished.  At  any  rate,  at  the 
present  time,  no  marine  crustacean  with  the  characters 
of  the  Astacidce  is  known. 

As  crayfishes  have  been  found  in  the  later  tertiariea 
of  North  America,  we  shall  hardly  err  in  dating  the 
existence  of  these  marine  crayfishes  at  least  as  far  back 
as  the  miocene  epoch ;  and  I  am  disposed  to  think  that, 
during  the  earlier  tertiary  and  later  mesozoic  periods, 
these  Crustacea  not  only  had  as  wide  a  distribution  as 
the  Prawns  and  Pencei  have  now,  but  were  differentiated 
into  two  groups,  one  with  the  general  characters  of  the 


332      DISTRIBUTION  AND  .ETIOLOGY   OF   THE  CRAYFISHES. 

Potamobiida  in  the  northern  hemisphere,  and  another, 
tfith  those  of  the  Parastacida,  in  the  southern  hemisphere. 

The  ancestral  Potaniobine  form  probably  presented 
the  peculiarities  of  the  Potamobiida  in  a  less  marked 
degree  than  any  existing  species  does.  Probably  the 
four  pleurobranchise  were  all  equally  well  developed  ;  the 
laminaB  of  the  podobranchJse  smaller  and  less  distinct 
from  the  stem ;  the  first  and  second  abdominal  appen- 
dages less  specialised ;  and  the  telson  less  distinctly 
divided.  So  far  as  the  type  was  less  specially  Pota- 
mobine, it  must  have  approached  the  common  form  in 
which  Homarus  and  Nephrops  originated.  And  it  is 
to  be  remarked  that  these  also  are  exclusively  confined 
to  the  northern  hemisphere. 

The  wide  range  and  close  affinity  of  the  genera 
Astacus  and  Cambarus  appear  to  me  to  necessitate  the 
supposition  that  they  are  derived  from  some  one  already 
specialised  Potamobine  form  ;  and  I  have  already  men- 
tioned the  grounds  upon  which  I  am  disposed  to  believe 
that  this  ancestral  Potamobine  existed  in  the  sea  which 
lay  north  of  the  miocene  continent  in  the  northern 
hemisphere. 

In  the  marine  primitive  crayfishes  south  of  the  equator, 
the  branchial  apparatus  appears  to  have  suffered  less 
modification,  while  the  suppression  of  the  first  abdominal 
appendages,  in  both  sexes,  has  its  analogue  among  the 
Pal'uiuridte,  the  headquarters  of  which  are  in  the 
southern  hemisphere.  That  they  should  have  ascended 


DISTRIBUTIONAL  DIFFICULTIES.  333 

the  rivers  of  New.  Zealand,  Australia,  Madagascar,  and 
South  America,  and  become  fresh  water  Parastacida,  is 
an  assumption  which  is  justified  by  the  analogy  of  the 
fresh-water  prawns.  It  remains  to  be  seen  whether 
maime  Parastacidce  still  remain  in  the  South  Pacific 
and  Atlantic  Oceans,  or  whether  they  have  become 
extinct. 

In  speculating  upon  the  causes  of  an  effect  which  is 
the  product  of  several  co-operating  factors,  the  nature 
of  each  of  which  has  to  be  divined  by  reasoning  back- 
wards from  its  effects,  the  probability  of  falling  into 
error  is  very  great.  And  this  probability  is  enhanced 
when,  as  in  the  present  case,  the  effect  in  question 
consists  of  a  multitude  of  phenomena  of  structure  and 
distribution  about  which  much  is  yet  imperfectly  known. 
Hence  the  preceding  discussion  must  rather  be  regarded 
as  an  illustration  of  the  sort  of  argumentation  by  which 
a  completely  satisfactory  theory  of  the  etiology  of  the 
crayfish  will  some  day  be  established,  than  as  sufficing 
to  construct  such  a  theory.  It  must  be  admitted  that 
it  does  not  account  for  the  whole  of  the  positive  facts 
which  have  been  ascertained;  and  that  it  requires  sup- 
plementing, in  order  to  furnish  even  a  plausible  explana- 
tion of  various  negative  facts. 

The  positive  fact  which  presents  a  difficulty  is  the 
closer  resemblance  between  the  Amur-Japanese  crayfish 
and  the  East  American  Cambari,  than  between  the 


334    '  DISTRIBUTION  AND  .ETIOLOGY  OF  THE  CRAYFISHES. 

latter  and  the  West  American  Astaci;  and  the  closer 
resemblance  between  the  latter  and  the  Pontocaspian 
crayfish,  than  either  bear  to  the  Amur-Japanese  form. 
If  the  facts  had  been  the  other  way,  and  the  West 
American  and  Amur-Japanese  crayfish  had  changed 
places,  the  case  would  have  been  intelligible  enough. 
The  primitive  Potamobine  stock  might  then  have  been 
supposed  to  have  differentiated  itself  into  a  western 
astacoid,  and  an  eastern  cambaroid  form ;  *  the  latter 
would  have  ascended  the  American,  and  the  former  the 
Asiatic  rivers.  As  the  matter  stands,  I  do  not  see  that 
any  plausible  explanation  can  be  offered  without  recourse 
to  suppositions  respecting  a  former  more  direct  com- 
munication between  the  mouth  of  the  Amur,  and  that 
of  the  North  American  rivers,  in  favour  of  which  no 
definite  evidence  can  be  offered  at  present. 

The  most  important  negative  fact  which  remains  to 
be  accounted  for  is  the  absence  of  crayfishes  in  the 
rivers  of  a  large  moiety  of  the  continental  lands,  and  in 
numerous  islands.  Differences  of  climatal  conditions  are 
obviously  inadequate  to  account  for  the  absence  of  cray- 
fishes in  Jamaica,  when  they  are  present  in  Cuba;  for 
their  absence  in  Mozambique,  and  the  islands  of  Johanna 
and  Mauritius,  when  they  are  present  in  Madagascar ; 
and  for  their  absence  in  the  Nile,  when  they  exist  in 
Guatemala. 

*  Just  as  there  is  an  American  form  of  Idothea  and  an  Asiatic  form 
in  the  Arctic  ocean  at  the  present  day. 


DISTRIBUTIONAL  DIFFICULTIES.  335 

At  present,  I  confess  that  I  do  not  see  my  way  to  s 
perfectly  satisfactory  explanation  of  the  absence  of  cray- 
fishes in  so  many  parts  of  the  world  in  which  they 
might,  d  priori,  be  expected  to  exist ;  and  I  can  only 
suggest  the  directions  in  which  an  explanation  may  be 
sought. 

The  first  of  these  is  the  existence  of  physical  obstacles 
to  the  spread  of  crayfishes,  at  the  time  at  which  the 
Potamobine  and  the  Parastacine  stocks  respectively  began 
to  take  possession  of  the  rivers,  some  of  which  have 
now  ceased  to  exist ;  and  the  second  is  the  probability 
that,  in  many  rivers  which  have  been  accessible  to  cray- 
fishes, the  ground  was  already  held  by  more  powerful 
competitors. 

If  the  ancestors  of  the  Potamobine  crayfishes  originated 
only  among  those  primitive  crayfishes  which  inhabited  the 
seas  north  of  the  miocene  continent,  their  present  limita- 
tion to  the  south,  in  the  old  world,  is  as  easily  intelligible 
as  is  their  extension  southward,  in  the  course  of  the  river 
basins  of  Northern  America  as  far  as  Guatemala,  but 
no  further.  For  the  elevation  of  the  Eurasiatic  high- 
lands had  commenced  in  the  miocene  epoch,  while  the 
isthmus  of  Panama  was  interrupted  by  the  sea. 

\Vith  respect  to  the  Southern  hemisphere,  the  absence 
of  crayfishes  in  Mauritius  and  in  the  islands  of  the  Indian 
Ocean,  though  they  occur  in  Madagascar,  may  be  due 
to  the  fact  that  the  former  islands  are  of  comparatively 
late  volcanic  origin ;  while  Madagascar  is  the  remnant  of 


336      DISTRIBUTION  AND  AETIOLOGY  OF  THE  CRAYFISHES. 

a  very  ancient  continental  area,  the  oldest  indigenous 
population  of  which,  in  all  probability,  is  directly  de- 
scended from  that  which  occupied  it  at  the  beginning 
of  the  tertiary  epoch.  If  Parastacine  Crustacea  inhabited 
the  southern  hemisphere  at  this  period ,  and  subsequently 
became  extinct  as  marine  animals,  their  preservation  in 
the  freshwaters  of  Australia,  New  Zealand,  and  the  older 
portions  of  South  America  may  be  understood.  The 
difficulty  of  the  absence  of  crayfishes  in  South  Africa  * 
remains  ;  and  all  that  can  be  said  is,  that  it  is  a  difficulty 
of  the  same  nature  as  that  which  confronts  us  when  we 
compare  the  fauna  of  South  Africa  in  general  with  that 
of  Madagascar.  The  population  of  the  latter  region  has 
a  more  ancient  aspect  than  that  of  the  former;  and  it 
may  be  that  South  Africa,  in  its  present  shape,  is  of  very 
much  later  date  than  Madagascar. 

With  respect  to  the  second  point  for  consideration,  it 
is  to  be  remarked  that,  in  the  temperate  regions  of  the 
world,  the  crayfishes  are  by  far  the  largest  and  strongest 
of  any  of  the  inhabitants  of  freshwater,  except  the  Verte- 
brata ;  and  that  while  frogs  and  the  like  fall  an  easy  prey 
to  them,  they  must  be  formidable  enemies  and  com- 
petitors even  to  fishes,  aquatic  reptiles,  and  the  smaller 
aquatic  mammals.  In  warm  climates,  however,  not  only 
the  large  prawns  which  have  been  mentioned,  but  Atyce 

*  But  it  must  be  remembered  that  we  have  as  yet  everything-  to  learn 
respecting  the  fauna  of  the  great  inland  lakes  and  river  systems  of 
South  Africa. 


THE  DISTRIBUTION   OF   CRABS  AND   CRAYFISHES.      337 

and  fluviatile  crabs  (Thelphusa)  compete  for  the  posses- 
sion of  the  freshwaters ;  and  it  is  not  improbable  that 
under  some  circumstances,  they  may  be  more  than  a 
match  for  crayfishes;  so  that  the  latter  might  either  be 
driven  out  of  territory  they  already  occupied,  as  Astacus 
leptodactylus  is  driving  out  A.  nobilis  in  the  Russian 
rivers;  or  might  be  prevented  from  entering  rivers  already 
tenanted  by  their  rivals. 

In  connection  with  this  speculation,  it  is  worthy  of 
remark  that  the  area  occupied  by  the  fluviatile  crabs  is 
very  nearly  the  same  as  that  zone  of  the  earth's  surface, 
from  which  crayfish  are  excluded,  or  in  which  they  are 
scanty.  That  is  to  say,  they  are  found  in  the  hotter 
parts  of  the  eastern  side  of  the  two  Americas,  the  West 
Indies,  Africa,  Madagascar,  Southern  Italy,  Turkey  and 
Greece,  Hindostan,  Burmah,  China,  Japan,  and  the 
Sandwich  Islands.  The  large-clawed  fluviatile  prawns 
are  found  in  the  same  regions  of  America,  on  both 
east  and  west  coasts,  in  Africa,  Southern  Asia,  the 
Moluccas,  and  the  Philippine  Islands  ;  while  the  Atyida 
not  only  cover  the  same  area,  but  reach  Japan,  extend 
over  Polynesia,  to  the  Sandwich  Islands,  on  the  north, 
and  New  Zealand,  on  the  south,  and  are  found  on  both 
shores  of  the  Mediterranean ;  a  blind  form  (Troglocaris 
Schniidtii),  in  the  Adelsberg  caves,  representing  the  blind 
Cambarus  of  the  caves  of  Kentucky. 

The   hypothesis  respecting    the   origin   of  crayfishes 

23 


338    DISTRIBUTION  AND  .ETIOLOGY  OF  THE   CRAYFISHES. 

which  has  been  tentatively  put  forward  in  the  preceding 
pages,  involves  the  assumption  that  marine  Crustacea  of 
the  astacine  type  were  in  existence  during  the  deposition 
of  the  middle  tertiary  formations,  when  the  great  con- 
tinents began  to  assume  their  present  shape.  That 
such  was  the  case  there  can  be  no  doubt,  inasmuch  as 
abundant  remains  of  Crustacea  of  that  type  occur  still 
earlier  in  the  mesozoic  rocks.  They  prove  the  existence 
of  ancient  crustaceans,  from  which  the  crayfishes  may  have 
been  derived,  at  that  period  of  the  earth's  history  when 
the  conformation  of  the  land  and  sea  were  such  as  to 
admit  of  their  entering  the  regions  in  which  we  now  find 
them. 

The  materials  which  have,  up  to  the  present,  time  been 
collected  are  too  scanty  to  permit  of  the  tracing  out  of  all 
the  details  of  the  genealogy  of  the  crayfish.  Nevertheless, 
the  evidence  which  exists  is  perfectly  clear,  as  far  as  it 
goes,  and  is  in  complete  accordance  with  the  require- 
ments of  the  doctrine  of  evolution. 

Mention  has  been  made  of  the  close  affinity  between 
the  crayfishes  and  the  lobsters — the  Astacina  and  the  IIo- 
marina ;  and  it  fortunately  happens  that  these  two  groups, 
which  may  be  included  under  the  common  name  of  the 
Astacomorpha,  are  readily  distinguishable  from  all  the 
other  Podophthalmia  by  peculiarities  of  their  exoskeleton 
which  are  readily  seen  in  all  well-preserved  fossils. 
In  all,  as  in  the  crayfish,  there  are  large  forceps,  fol- 
lowed by  two  pairs  of  chelate  ambulatory  limbs,  while 


FOSSIL  ASTACOMORPHA.  339 

the  succeeding  two  pairs  of  legs  are  terminated  by  simple 
claws.  The  exopodite  of  the  last  abdominal  appendage 
is  divided  into  two  parts  by  a  transverse  suture.  The 
pleura  of  the  second  abdominal  somite  are  larger  than 
the  others,  and  overlap  those  of  the  first  somite, 
which  are  very  small.  Any  fossil  crustacean  which 
presents  all  these  characters,  is  certainly  one  of  the 
Astacomorpha. 

The  Astacina,  again,  are  distinguished  from  the  Hoina- 
rina  by  the  mobility  of  the  last  thoracic  somite,  and  the 
characters  of  the  first  and  second  abdominal  appendages, 
when  they  are  present ;  or  by  their  entire  absence. 
But  it  is  so  difficult  to  make  out  anything  about  either 
of  these  characters  in  fossils,  that,  so  far  as  I  am  aware, 
we  know  nothing  about  them  in  any  fossil  Astacomorph. 
And  hence,  it  may  be  impossible  to  say  to  which  division 
any  given  form  belongs,  unless  its  resemblances  tc 
known  types  are  so  minute  and  so  close  as  to  remove 
doubt. 

For  the  present  purpose,  the  series  of  the  fossiliferous 
rocks  may  be  grouped  as  follows : — 1.  Recent  and 
Quaternary.  2.  Newer  Tertiary  (Pliocene  and  Miocene). 
3.  Older  Tertiary  (Eocene).  4.  Cretaceous  (Chalk, 
Greensand  and  Gault).  5.  Wealden.  6.  Jurassic  (Pur- 
beck  to  Inferior  Oolite).  7.  Liassic.  8.  Triassic.  9. 
Permian.  10.  Carboniferous.  11.  Devonian.  12. 
Silurian.  13.  Cambrian. 

Now  the  oldest  known  member  of  the  group  of  the 


PIG.  80. — A,  Psevdastacus  pustulosus  (nat.  size).     B,  Eryma  modesti 
for  mis  (  x  2).    Both  figures  after  OppeL 


THE   EXTINCT  GENUS  ERYMA.  341 

decapod  Podophthalmia  to  which  the  Astacomorpha  belong 
occurs  in  the  Carboniferous  formation.  It  is  the  gi  nus 
Anthrnp  damon — a  small  and  very  curious  crustacean, 
about  which  nothing  more  need  be  said  at  present,  as  it 
does  not  appear  to  have  special  affinities  with  the  Astaco- 
morplw.  In  the  later  formations,  up  to  the  top  of  the 
Trias,  podophthalmatous  Crustacea  are  very  rare ;  and, 
unless  the  Triassic  genus  Pcmphix  is  an  exception,  no 
Astacomorphs  are  known  to  occur  in  them.  The  speci- 
mens of  Pemphix  which  I  have  examined  are  not  suffi- 
ciently complete  to  enable  me  to  express  any  opinion 
about  them. 

The  case  is  altered  when  we  reach  the  Middle  Lias.  In 
fact  this  yields  several  forms  of  a  genus,  Eryma  (fig.  80,  B), 
which  also  occurs  in  the  overlying  strata  almost  up  to  the 
top  of  the  Jurassic  series,  and  presents  so  many  variations 
that  nearly  forty  different  species  have  been  recognised. 
Eryma  is,  in  all  respects,  an  Astacomorph,  and  so  far  as 
can  be  seen,  it  differs  from  the  existing  genera  only 
in  such  respects  as  those  in  which  they  differ  from 
one  another.  Thus  it  is  quite  certain  that  Astacomor- 
phous  Crustacea  have  existed  since  a  period  so  remote 
as  the  older  part  of  the  Mesozoic  period ;  and  any  hesi- 
tation in  admitting  this  singular  persistency  of  type  on 
the  part  of  the  crayfishes,  is  at  once  removed  by  the 
consideration  of  the  fact  that,  along  with  Eryma,  in  thf 
Middle  Lias,  prawn-like  Crustacea ,  generically  iden- 
tical with  the  existing  Penceus,  flourished  in  the  sea 


34-2   DISTRIBUTION   AND  AETIOLOGY  OF  THE  CRAYFISHES. 

and  left   their   remains   in  the  mud  of  the  ancient  sea 
bottom. 

Eryma  is  the  only  crustacean,  which  can  be  certainly 
ascribed  to  the  Astacomorpha,  that  has  hitherto  been 
found  in  the  strata  from  the  Middle  Lias  to  the  litho- 
graphic slates ;  which  last  lie  in  the  upper  part  of  the 
Jurassic  series.  In  the  freshwater  beds  of  the  Wealden, 
no  Astacomor2)ha  are  known,  and  although  no  very  great 


FIG.  81.  —  Hoploparia  longimana  (f  nat.  size).  —  cp,  carapace; 
r,  rostrum,  T,  telson  ;  xv.,  xvi.,  first  and  second  abdominal  somites  \ 
10,  forceps  ;  20,  last  abdominal  appendage. 


weight  is  to  be  attached  to  a  negative  fact  of  this  kind,  it 
is,  so  far,  evidence  that  the  Astacomorpha  had  not  yet 
taken  to  freshwater  life.  In  the  marine  deposits  of  the 
Cretaceous  epoch,  however,  astacomorphous  forms,  which 


HOPLOPARIA  AND   PSEUD  AST  ACUS.  343 

are  known  by  the  generic  names  of  Hoploparia  and 
Enoploclytia,  are  abundant. 

The  differences  between  these  two  genera,  and  between 
both  and  Eryma,  are  altogether  insignificant  from  a  broad 
morphological  point  of  view.  They  appear  to  me  to  be 
of  less  importance  than  those  which  obtain  between  the 
different  existing  genera  of  crayfishes. 

Hoploparia  is  found  in  the  London  clay.  It  therefore 
extends  beyond  the  bounds  of  the  Mesozoic  epoch  into 
the  older  Tertiary.  But  when  this  genus  is  compared 
with  the  existing  Homarus  and  Nephrops,  it  is  found 
partly  to  resemble  the  one  and  partly  the  other.  Thus, 
on  one  line,  the  actual  series  of  forms  which  have 
succeeded  one  another  from  the  Liassic  epoch  to  the 
present  day,  is  such  as  must  have  existed  if  the  common 
lobster  and  the  Norway  lobster  are  the  descendants  of 
Erymoid  crustaceans  which  inhabited  the  seas  of  the 
Liassic  epoch. 

Side  by  side  with  Eryma,  in  the  lithographic  slates, 
there  is  a  genus,  Pseudastacus  (fig.  80,  A),  which,  as  its 
name  implies,  has  an  extraordinarily  close  resemblance  to 
the  crayfishes  of  the  present  day.  Indeed  there  is  no  point 
of  any  importance  in  which  (in  the  absence  of  any  know- 
ledge of  the  abdominal  appendages  in  the  males)  it  differs 
from  them.  On  the  other  hand,  in  some  features,  as  in  the 
structure  of  the  carapace,  it  differs  from  Eryma,  much 
as  the  existing  crayfishes  differ  from  Nephrops.  Thus,  in 
the  latter  part  of  the  Jurassic  epoch,  the  Astacine  type 


344   DISTRIBUTION   AND   .ETIOLOGY  OF  THE  CRAYFISHES. 

was  already  distinct  from  the  Homarine  type,  though 
both  were  marine;  and,  since  Eryma  begins  at  least 
as  early  as  the  Middle  Lias,  it  is  possible  that  Pseudas* 
tacus  goes  back  as  far,  and  that  the  common  protas- 
tacine  form  is  to  be  sought  in  the  Trias.  Pseudasta&is 
is  found  in  the  marine  cretaceous  rocks  of  the  Leba- 
non, but  has  not  yet  been  traced  into  the  Tertiary 
formations. 

I  am  disposed  to  think  that  Pseudastacus  is  comparable 
to  such  a  form  as  Astacus  nigrescens  rather  than  to  any 
of  the  Parastacidce,  as  I  doubt  the  existence  of  the  latter 
group  at  any  time  in  northern  latitudes. 

In  the  chalk  of  Westphalia  (also  a  marine  deposit)  a 
single  specimen  of  another  Astacomorph  has  been  dis- 
covered, which  possesses  an  especial  interest  as  it  is 
a  true  Astacus  (A.  politus,  Von  der  Marck  and  Schliiter), 
provided  with  the  characteristic  transversely  divided 
telson  which  is  found  in  the  majority  of  the  Pota- 
mobiidce. 

If  we  arrange  the  results  of  palseontological  inquiry 
which  have  now  been  stated  in  the  form  of  a  table 
such  as  that  which  is  given  on  the  following  page, 
the  significance  of  the  succession  of  astacomorphoua 
forms,  in  time,  becomes  apparent. 


THE  GENEALOGY  OF  THE  CRAYFISHES. 


345 


SUCCESSIVE  FORMS  OP  THE  ASTACOMORPHOUS  TYPE. 

i    Recent  Potamobiidce.  Homarina. 


Penccut. 


IL 

Later  Tertiary 

Aftacus 
(Idaho). 

nt 

Earlier  Tertiary. 

HopU 

tparia. 

IT. 

Cretaceous 

AH, 

zus.   Pseudastacus.       Enoplodytia.   Hoploparia. 

v.  Wealden                                  \                                     / 
(Fresh  Water).                        \                                 / 

\                         / 

n.  Jurassic.                               Pseudastacus    Eryma. 

• 
Pen  CPU* 

\                 1 
vn.   Liassio.                                                           Eryma. 

Penaxu. 

TIII.  Triassic. 

*T.  Permian. 

TL.  Carboniferous.                            Anihrapalcemon 

xi.  Devonian. 

xn.  Silurian. 

VIIL  Cambrian, 

If  an  Astacomorphous  crustacean,  having  characters 
intermediate  between  those  of  Eryma  and  those  of 
Pseudastacus,  existed  in  the  Triassic  epoch  or  earlier; 
if  it  gradually  diverged  into  Pseudastacine  and  Erymoid 
forms;  if  these  again  took  on  Astacine  and  Homarine 


346   DISTRIBUTION   AND   .ETIOLOGY   OF   THE   CRAYFISHES. 

characters,  and  finally  ended  in  the  existing  Potamobiida 
and  Homarina,  the  fossil  forms  left  in  the  track  of  this 
process  of  evolution  would  be  very  much  what  they 
actually  are.  Up  to  the  end  of  the  Mesozoic  epoch 
the  only  known  Potamobiidae  are  marine  animals.  And 
we  have  already  seen  that  the  facts  of  distribution 
suggest  the  hypothesis  that  they  must  have  been  so, 
at  least  up  to  this  time. 

Thus,  with  respect  to  the  ^Etiology  of  the  crayfishes, 
all  the  known  facts  are  in  harmony  with  the  requirements 
of  the  hypothesis  that  they  have  been  gradually  evolved 
in  the  course  of  the  Mesozoic  and  subsequent  epochs 
of  the  world's  history  from  a  primitive  Astacomorphous 
form. 

And  it  is  well  to  reflect  that  the  only  alternative  sup- 
position is,  that  these  numerous  successive  and  coexistent 
forms  of  insignificant  animals,  the  differences  of  which 
require  careful  study  for  their  discrimination,  have  been 
separately  and  independently  fabricated,  and  put  into  the 
localities  in  which  we  find  them.  By  whatever  verbal  fog 
the  question  at  issue  may  be  hidden,  this  is  the  real 
nature  of  the  dilemma  presented  to  us  not  only  by  the 
crayfish,  but  by  every  animal  and  by  every  plant ;  from 
man  to  the  humblest  animalcule  ;  from  the  spreading 
beech  and  towering  pine  to  the  Micrococci  which  lie  at 
the  limit  of  microscopic  visibility. 


NOTES. 


NOTE  1.,  CHAPTER  L,  p.  17. 
THE  CHEMICAL  COMPOSITION  OF  TB1C  EXOSKELETOff. 

THE  harder  parts  of  the  exoskeleton  of  the  crayfish  contain  rathei 
m-  re  than  half  their  weight  of  calcareous  salts.  Of  these  nearly 
seven-eighths  consist  of  carbonate  of  lime,  the  rest  being  phosphate  ol 
litre. 

The  animal  matter  consists  for  the  most  part  of  a  peculiar  substance 
termed  Chitin,  which  enters  into  the  composition  of  the  hard  parts  not 
only  of  the  Arthropoda  in  general  but  of  many  other  invertebrated 
animals.  Chitin  is  not  dissolved  even  by  hot  caustic  alkalies,  whence 
the  use  <>f  solutions  of  caustic  potash  and  soda  in  cleaning  the  skeletons 
of  crayfishes.  It  is  soluble  in  cold  concentrated  hydrochloric  acid  with- 
out change,  and  may  be  precipitated  from  its  solution  by  the  addition  of 
water. 

Chitin  contains  nitrogen,  and  according  to  the  latest  investigations 
(Ledderhose,  "Ueber  Chitin  und  seine  Spaltungs-produkte  : "  Zeitschrift 
fiir  Physiologische  Chemie,  II.  1879)  its  composition  is  represented  by  the 
formula  Clf  H.M  N,  O10. 


NOTE  II.,  CHAPTER  L,  p.  29. 
THE  CRIB'S  EYES,  OR  GASTROLITHS. 

The  "  Gastroliths,"  as  the  "  crab's  eyes  "  may  be  termed,  are  found 
folly  developed  only  in  the  latter  part  of  the  summer  season,  just  before 
ccdysis  sets  in.  They  then  give  rise  to  rounded  prominences,  one  on 


348  NOTES. 

each  side  of  the  anterior  part  of  the  cardiac  division  of  the  stomach.  The 
proper  wall  of  the  stomach  is  continued  over  the  outer  surface  of  the 
prominence  ;  and,  in  fact,  forms  the  outer  wall  of  the  chamber  in  which 
the  gastroiith  is  contained,  the  inner  wall  being  formed  by  the  cuti- 
cular  lining  of  the  stomach.  When  the  outer  wall  is  cut  through,  it  ia 
readily  detached  from  the  convex  outer  surface  of  the  gastroiith,  with 
which  it  is  in  close  contact.  The  inner  surface  of  the  gastroiith  is  usually 
flat  or  slightly  concave.  Sometimes  it  is  strongly  adherent  to  the  chi- 
tonous  cuticula  ;  but  when  fully  formed  it  is  readily  detached  fn.  i  the 
latter. '  Thus  the  proper  wall  of  the  stomach  invests  only  the  outer  face 
of  the  gastroiith,  the  inner  face  of  which  is  adherent  to,  or  at  any  rate  in 
close  contact  with,  the  cuticula.  The  gastroiith  is  by  no  means  a  mere 
concretion,  but  is  a  cuticular  growth,  having  a  definite  structure.  Its 
inner  surface  is  smooth,  but  the  outer  surface  is  rough,  from  the  projec- 
tion of  irregular  ridges  which  form  a  kind  of  mesh  work.  A  vertical  sec- 
tion shows  that  it  is  composed  of  thin  superimposed  layers,  of  which  the 
inner  are  parallel  with  the  flat  inner  surface,  while  the  outer  becomes 
gradually  concentric  with  the  outer  surface.  Moreover,  the  inner  layers 
are  less  calcified  than  the  outer,  the  projections  of  the  outer  surface  being 
particularly  dense  and  hard.  In  fact,  the  gastroliths  are  very  similar  to 
other  hard  parts  of  the  exoskeleton  in  structure,  except  that  the  densest 
layers  are  nearest  the  epithelial  substratum,  instead  of  furthest  away 
from  it. 

When  ecdysis  occurs,  the  gastroliths  are  cast  off  along  with  the  gas* 
trie  armature  in  general,  into  the  cavity  of  the  stomach,  and  are  there 
dissolved,  a  new  cuticle  being  formed  external  to  them  from  the  proper 
wall  of  the  stomach.  The  dissolved  calcareous  matter  is  probably  used 
up  in  the  formation  of  the  new  exoskeleton. 

According  to  the  observations  of  M.  Chantran  (Comptes  Rendus, 
LXXVIII.  1874)  the  gastroliths  begin  to  be  formed  about  forty  days 
before  ecdysis  takes  place  in  crayfish  of  four  years'  old  ;  but  the 
interval  is  less  in  younger  crayfish,  and  is  not  more  than  ten  days 
during  the  first  year  after  birth.  When  shed  into  the  stomach  during 
ecdysis  they  are  ground  down,  not  merely  dissolved.  The  process 
of  destruction  and  absorption  takes  twenty-four  to  thirty  hours 
in  very  young  crayfish,  seventy  to  eighty  hours  in  adults.  Unless 
the  gastroliths  are  normally  developed  and  re- absorbed,  ecdysis  ia 
not  healthily  effected,  and  the  crayfish  dies  in  the  course  of  tna 
process. 


NOTES. 


349 


According  to  Bulk ("Chemische  UntersuchungderKrebsteine:"  MUller'a 
Archiv.  1835),  the  gastroliths  have  the  following  composition  : — 
Animal  matter  soluble  in  water         .        .         .        .11*43 
Animal  matter  insoluble  in  water  (probably  chitin)       4-33 

Phosphate  of  lime 18-60 

Carbonate  of  lime 63-16 

Soda  reckoned  as  carbonate      .        .        •        .        .      1*41 

98-93 

The  proportion  of  mineral  to  animal  matter  and  of  phosphate  to  car- 
bonate of  lime  is  therefore  greater  in  the  gastroliths  than  in  the  exo- 
skeleton  in  general. 


NOTE  HI.,  CHAPTER  L,  p.  31. 
GROWTH  OF  CRAYFISH. 

The  statements  in  the  text,  after  the  words  "  By  the  end  of  the  year," 
regarding  the  sizes  of  the  crayfish  at  different  ages,  are  given  on  the 
authority  of  M.  Carbonnier  (L'Ecrevisse.  Paris,  1869)  ;  but  they  obviously 
apply  only  to  the  large  "Ecrevisse  a  pieds  rouges"  of  France,  and  not  to 
the  English  crayfish,  which  appears  to  be  identical  with  the  "  Ecrevisse 
a  pieds  blancs,"  and  is  of  much  smaller  size.  According  to  M.  Carbonnier 
(1.  c.  p.  51),  the  young  crayfish  just  born  is  "  un  centimetre  et  demi 
environ,"  that  is  to  say,  three-fifths  of  an  inch  long.  The  young  of  the 
English  crayfish  still  attached  to  the  mother,  which  I  have  seen,  rarely 
exceeds  half  this  length. 

M.  Soubeiran  ("Sur  1'histoire  naturelle  et  1'education  des  iWevisses:" 
Comptes  Rendus,  LX.  1865)  gives  the  result  of  his  study  of  the  growth 
of  the  crayfishes  reared  at  Clairefontaine,  near  Rambouillet,  in  the 
following  table : 


Crayfish  of  the  year 

1  year  old 

2  years  old 

3  years  „ 

4  years  „ 

5  years  „ 
indeterminate 
very  old 


Mean  length. 
Metres. 
0-025 
0-050 
0-070 
0-090 
0-110 
0-125 
0-160 
0-190 


Mean  weight. 
Grammes. 

0-50 

1-60 

3-50 

6-50 

17-50 

18-50 

30-00 

125-00 


These  observations  must  also  apply  to  the  "  Ecrevisse  a  pieds  rouges." 


350  NOTES. 


NOTE  IV.,  CHAPTER  I,  p.  37. 
THE   ECDYSES  OF   CRAYFISHES. 

There  is  a  good  deal  of  discrepancy  between  different  observers  as  t* 
the  frequency  of  the  process  of  ecdysis  in'  crayfishes.  In  the  text  I  have 
followed  M.  Carbonnier,  but  M.  Chantran  ("  Observations  sur  1'histoire 
natureUedes  Ecrevisses  :"  Comptes  Rendus,  LXXI.  1870,  and  LXXII1. 
1871),  who  appears  to  have  studied  the  question  (on  the  "ecrevisse 
a  pieds  rouges "  apparently)  very  carefully,  declares  that  the  young 
crayfish  moults  no  fewer  than  eight  times  in  the  course  of  the  first  twelve 
months.  The  first  moult  takes  place  ten  days  after  it  is  hatched  ;  the 
second,  third,  fourth,  and  fifth,  at  intervals  of  from  twenty  to  twenty-five 
days,  so  that  the  young  animal  moults  five  times  in  the  course  of  the 
ninety  to  one  hundred  days  of  July,  August,  and  September.  From  the 
latter  month  to  the  end  of  April  in  the  following  year,  no  ecdysis  takes 
place.  The  sixth  takes  place  in  May,  the  seventh  in  June,  and  the  eighth 
in  July.  In  the  second  year  of  its  age,  the  crayfish  moults  five  times,  that 
is  to  say,  in  August  and  in  September,  and  in  May,  June,  and  July 
following.  In  the  third  year,  the  crayfish  commonly  moults  only  twice, 
namely  in  July  and  in  September.  At  a  greater  age  than  this,  the 
females  moult  only  once  a  year,  from  August  to  September ;  while  the 
males  moult  twice,  first  in  June  and  July  ;  afterwards  in  August  and 
September. 

The  details  of  the  process  of  ecdysis  are  discussed  by  Braun,  "  Ueber 
die  histologischen  Yorgange  bei  der  Hautung  von  Astacus  fluviatilia." 
Wiirzburg  Arbeiten,  Bd.  II. 


NOTE  V.,  CHAPTER  I.,  p.  39. 
REPRODUCTION  IN  CRAYFISHES. 

The  males  are  said  to  approach  the  females  in  November,  December, 
and  January,  in  the  case  of  the  French  crayfishes.  In  England  they 
certainly  begin  as  early  as  the  beginning  of  October,  if  not  earlier. 
According  to  M.  Chajitran  (Comptes  Rendus,  1870),  and  M.  Gerbe 
(Comptes  Rendus,  1 858),  the  male  seizes  the  female  with  his  pincers, 
throws  hei  on  her  back,  and  deposits  the  spermatic  matter,  firstly,  on  the 
external  plates  of  the  caudal  fin  ;  secondly,  on  the  thoracic  sterna  around 
the  external  openings  of  the  oviducts.  During  this  operation,  the 
appendages  of  the  two  first  abdominal  somites  are  carried  backwards, 


NOTES.  351 

the  extremities  of  the  posterior  pair  are  inclosed  in  the  groove  of  the 
anterior  pair ;  and  the  end  of  the  vas  deferens  becoming  everted  and 
prominent,  the  seminal  matter  is  poured  out,  and  runs  slowly  along  the 
groove  of  the  anterior  appendage  to  its  destination,  where  it  hardens  and 
assumes  a  vermicular  aspect.  The  filaments  of  which  it  is  composed  are, 
in  fact,  tubular  spermatophores,  and  consist  of  a  tough  case  or  sheath 
filled  with  seminal  matter.  The  spoon-shaped  extremity  of  the  second 
abdominal  appendage,  working  backwards  and  forwards  in  the  groove 
of  the  anterior  appendage,  clears  the  seminal  matter  out  of  it,  and 
prevents  it  from  becoming  choked. 

After  an  interval  -which  varies  from  ten  to  forty-five  days,  oviposition 
takes  place.  The  female,  resting  on  her  back,  bends  the  end  of  the 
abdomen  forward  over  the  hinder  thoracic  sterna,  so  that  a  chamber  is 
formed  into  which  the  oviducts  open.  The  eggs  are  passed  into  the 
chamber  by  one  operation,  usually  during  the  night,  and  are  plunged 
into  a  viscous  greyish  mucus  with  which  it  is  filled.  The  spermatozoa 
pass  out  of  the  vermicular  spermatophores,  and  mix  with  this  fluid,  in 
which  the  peculiarity  of  their  form  renders  them  readily  recognisable. 
The  spermatozoa  are  thus  brought  into  close  relation  with  the  ova,  but 
what  actually  becomes  of  them  is  unknown. 

The  origin  of  the  viscous  matter  which  fills  the  abdominal  chamber 
when  the  eggs  are  deposited  in  it,  and  the  manner  in  which  these  become 
fixed  to  the  abdominal  limbs  is  discussed  by  Lerebonllet  ("Recherches 
sur  le  mode  de  fixation  des  oeafs  aux  faux  pattes  abdominaux  dans  les 
Ecrevisses."  Annales  des  Sciences  Naturelles,  4e  Ee.  T.  XIV.  1860), 
and  by  Braun  (Arbeiten  aus  dem  Zoologisch-Zootomischen  Institut  in 
Wurzburg,  tL). 


NOTE  VI.,  CHAPTBB  L,  p.  42. 
ATTACHMENT  OF  THE  YOUNG  CRAYFISH  TO  THE  MOTHER. 

I  observe  that  I  had  overlooked  a  passage  in  the  Report  on  the 
award  of  the  Prix  Montyon  for  1872,  Comptes  Rendus,  LXXV.  p.  1341, 
in  which  M.  Chantran  is  stated  to  have  ascertained  that  the  young 
crayfishes  fix  themselves  "en  saisissaut  avec  un  de  leurs  pinces  le 
filament  qui  suspend  1'ceuf  a  une  fausse  paite  de  ia  mere." 

In  the  paper  already  cited  from  the  Comptes  Rendus  for  1870,  M.  Chan, 
tran  states  that  the  young  remain  attached  to  the  mother  during  ten  days 
after  hatching,  that  is  to  say,  up  to  the  first  moult.  Detached  before 
this  period,  they  die  ;  but  after  the  first  moult,  they  sometimes  leave  the 


352  NOTES. 

mother  and  return  to  her  again,  up  to  twenty -eight  days,  when  they 
become  independent. 

In  a  note  appended  to  M.  Chantran's  paper,  M.  Robin  states,  that  "  the 
young  are  suspended  to  the  abdomen  of  the  mother  by  the  intermediation 
of  a  chitinous  hyaline  filament,  which  extends  from  a  point  of  the 
internal  surface  of  the  shell  of  the  egg  as  far  as  the  four  most  in- 
ternal filaments  of  each  of  the  lobes  of  the  median  membranous  plate 
of  the  caudal  appendage.  The  filaments  exist  when  the  embryos  have 
not  yet  attained  three-fourths  of  their  development."  Is  this  a  larval 
coat  1  Rathke  does  not  mention  it  and  I  have  seen  nothing  of  it  in 
those  receutly  hatched  young  which  I  have  had  the  opportunity  of 
examining. 


NOTE  VII.,  CHAPTER  II.,  p.  64. 

THE  "SALIVARY"  GLANDS  AND  THE  SO-CALLED  "LIVER"  OP 
THE  CRAYFISH. 

Braun  (Arbeiten  aus  dem  Zoologisch-Zootomischen  Institut  in 
Wurzburg,  Bd.  II.  and  III.)  has  described  "salivary"  glands  in  the 
walls  of  the  oesophagus,  in  the  metastoma,  and  in  the  first  pair  of  maxillae 
of  the  crayfish. 

Hoppe-Seyler  (Pfliigers  Archiv,  Bd.  XIV.  1877)  finds  that  the  yellow 
fluid  ordinarily  found  in  the  stomachs  of  crayfishes  always  contains  pep- 
tone. It  dissolves  fibrin  readily,  without  swelling  it  up,  at  ordinary  tem- 
peratures ;  more  quickly  at  40°  Centigrade.  The  action  is  delayed  by  even 
a  trace  of  hydrochloric  acid,  and  is  stopped  by  the  addition  of  a  few  drops 
of  water  containing  0.2  per  cent,  of  that  acid.  By  adding  alcohol  to  the 
yellow  fluid,  a  precipitate  is  obtained,  which  is  soluble  in  water  and  in 
glycerine.  The  aqueous  solution  of  the  precipitate  has  a  strong  digestive 
action  on  fibrin,  which  is  arrested  by  acidulation  with  hydrochloric  acid. 
These  reactions  show  that  the  fluid  is  very  similar  to,  if  not  identical 
with,  the  pancreatic  fluid  of  vertebrates. 

The  secretion  of  the  "  liver  "  taken  directly  from  that  gland,  has  a 
more  strongly  acid  reaction  than  the  fluid  in  the  stomach,  but  has 
similar  digestive  properties.  So  has  an  aqueous  extract  of  the  gland, 
and  a  watery  solution  of  the  alcoholic  precipitate.  The  aqueous  extract 
also  possesses  a  strong  diastatic  action  on  starch,  and  breaks  up  olive  oil, 
There  is  no  more  glycogen  in  the  "  liver  "  than  is  to  be  found  in  o^hei 
organs,  and  no  constituents  of  true  bile  are  to  be  met  with. 


NOTES.  363 


NOTE  VIII.,  CHAPTER  IL,  p.  81. 
ANAL  RESPIEATION  IN  CRAYFISH. 

Lereboullet  ("  Note  sur  une  respiration  anale  observee  chez  plusieurs 
Crustaces  ; "  Memoires  de  la  Societe  d'Histoire  Naturelle  de  Strasbourg, 
IV.  1850)  has  drawn  attention  to  what  he  terms  "  anal  respiration ''  in 
young  crayfish,  in  which  he  observed  water  to  be  alternately  taken  into 
and  expelled  from  the  rectum  fifteen  to  seventeen  times  in  a  minute. 
I  have  never  been  able  to  observe  anything  of  this  kind  in  the  uninjured 
adult  animal,  but  if  the  thoracic  ganglia  are  destroyed,  a  regular 
rhythmical  dilatation  and  closing  of  the  anal  end  of  the  rectum  at  once 
sets  in,  and  goes  on  as  long  as  the  hindermost  ganglia  of  the  abdomen 
retain  their  integrity.  I  am  much  disposed  to  imagine  that  the  rhyth- 
mical movement  is  inhibited,  when  the  uninjured  crayfish  is  held  in  such 
a  position  that  the  vent  can  be  examined. 


NOTE  IX.  CHAPTER  IL,  p.  82. 
THE  GREEN  GLAND. 

The  existence  of  guanin  in  the  green  gland  rests  on  the  authority 
of  Will  and  Gorup-Besanez  (Gelehrte  Anzeigen,  d.  k.  Baienzschen 
Akademie,  No.  233,  1848),  who  say  that  in  this  organ  and  in  the  organ  of 
Bo  janus  of  the  fresh  water  mussel,  they  found  "  a  substance  the  reactions 
of  which  with  the  greatest  probability  indicate  guanin,"  but  that  they 
had  been  unable  to  obtain  sufficient  material  to  give  decisive  results. 

Leydig  (Lehrbuch  der  Histologie,  p.  467)  long  ago  stated  that  the 
green  gland  consists  of  a  much  convoluted  tube  containing  granular  cells 
disposed  around  a  central  cavity.  Wassiliew  ("  Ueber  die  Niere  des 
Flusskrebses  :  "  Zoologischer  Anzeiger,  I.  1878)  supports  the  same  view, 
giving  a  full  account  of  the  minute  strucMire  of  the  organ,  and  com- 
paring it  with  its  homologues  in  the  Copepoda  and  Phyllopoda. 


NOTE  X.,  CHAPTER  in.,  p.  105. 

THE  ANATOMY  OF  THE  NERVOUS  SYSTEM  OP  THE 
CRAYFISH. 

The  details  respecting  the  origin  and  the  distribution  of  the  nerves  are 
intentionally  omitted.     See  the  memoir  by  Lemoine  of  which  the  title  is 
given  in  the  "  Bibliography." 
24 


354  NOTES. 

NOTE  XI.,  CHAPTER  III.,  p.  110. 

THE  FUNCTIONS  OF  THE  NERVOUS  SYSTEM  OF  THE 
CRAYFISH. 

Mr.  J.  Ward,  in  his  "  Observations  on  the  Physiology  of  the  Nervous 
System  of  the  Crayfish,"  (Proceedings  of  the  Royal  Society,  1879)  has 
given  an  account  of  a  number  of  interesting  and  important  experiments 
on  this  subject. 

NOTE  XII.,  CHAPTEE  III.  p.  124. 
THE   THEORY   OF   MOSAIC  VISION. 

Oscar  Schmidt  ("Die  Form  der  Krystalkegel  im  Arthropoden  Auge  :" 
Zeitschrift  fiir  Wissenschaftliche  Zoologie,  XXX.  1878)  has  pointed  out 
certain  difficulties  in  the  way  of  the  universal  application  of  the  theory  of 
mosaic  vision  in  its  present  form,  which  are  well  worthy  of  consideration. 
I  do  not  think,  however,  that  the  substance  of  the  theory  is  affected  by 
Schmidt's  objections. 


NOTE  XIII.,  CHAPTER  III.,  p.  135. 

THE  SPERMATOZOA. 

Since  the  discovery  of  the  spermatozoa  of  the  crayfish  in  1835-36  by 
Henle  and  von  Siebold.  the  structure  and  development  of  these  bodies 
have  been  repeatedly  studied.  The  latest  discussion  of  the  subject  is 
contained  in  a  memoir  of  Dr.  C.  Grobben  ("  Beitrage  zur  Kenntniss  der 
mannlichen  Geschlechtsorgane  der  Dekapoden  : "  Wien,  1878).  There 
is  no  doubt  that  the  spermatozoon  consists  of  a  flattened  or  hemi- 
spherical body,  produced  at  its  circumference  into  a  greater  or  less 
number  of  long  tapering  curved  processes  (fig.  34  F).  In  the  interior 
of  this  are  two  structures,  one  of  which  occupies  the  greater  part 
of  the  body,  and,  when  the  latter  lies  flat,  looks  like  a  double  ring. 
This  may  be  called,  for  distinctness'  sake,  the  annulate  corpuscle.  The 
other  is  a  much  smaller  oval  corpuscle,  which  lies  on  one  side  of  the 
first.  The  annulate  corpuscle  is  dense,  and  strongly  refracting  ;  the  oval 
corpuscle  is  soft,  and  less  sharply  defined.  Dr.  Grobben  describes  the 
annulate  corpuscle  as  "  napfartig,"  01  cup-shaped  ;  closed  below,  open 
above,  and  with  the  upper  edge  turned  inwards,  and  applied  to  the 
i  >ner  side  of  the  wall  of  the  cup.  It  appeared  to  me,  on  the  other 
...and,  that  the  annulate  corpuscle  is  really  a  "hollow  ring,  somewhat 


NOTES.  355 

like  one  of  the  ring-shaped  air-cushions  one  sees,  on  a  very  small  scale. 
Dr.  Grobben  describes  the  spermatoblastic  cells  of  the  testis  and  their 
nuclear  spindles  ;  but  his  accoant  of  the  development  of  the  spermatozoa 
does  not  agree  with  my  own  observations,  which,  so  far  as  they  have 
gone,  lead  me  to  infer  that  the  annulate  corpuscle  of  the  spermatozoon 
i&  the  metamorphosed  nucleus  of  the  cell  from  which  the  spermatozoon  is 
developed.  For  want  of  material,  however,  I  was  unable  to  bring  my 
investigations  to  a  satisfactory  termination,  and  I  speak  with  reserve. 


NOTE  XIV.,  CHAPTER  IV.,  p.  174. 
THE  MORPHOLOGY  OF  THE  CRAYFISH. 

The  founder  of  the  morphology  of  the  Crustacea,  M.  Milne  Edwards, 
counts  the  telson  as  a  somite,  and  consequently  considers  that  twenty- 
one  somites  enter  into  the  composition  of  the  body  in  the  PodopK- 
thalmia.  Moreover,  he  assigns  the  anterior  seven  somites  to  the  head,  the 
middle  seven  to  the  thorax,  and  the  hinder  seven  to  the  abdomen. 
There  is  a  tempting  aspect  of  symmetry  about  this  arrangement ;  but  as 
to  the  limits  of  the  head,  the  natural  line  of  demarcation  between  it  and 
the  thorax  seems  to  me  to  be  so  clearly  indicated  between  the  somite 
which  bears  the  second  maxillae  and  that  which  carries  the  first  maxilli- 
pedes  in  the  Crustacea,  wad.  between  the  homologous  somites  in  Insects,  that 
I  have  no  hesitation  in  retaining  the  grouping  which  I  have  for  many  years 
a  lopted.  The  exact  nature  of  the  telson  needs  to  be  elucidated,  but  I  can 
find  no  ground  for  regarding  it  as  the  homologue  of  a  single  somite. 

It  will  be  observed  that  ihese  differences  of  opinion  turn  upon  ques- 
tions of  grouping  and  nomenclature.  It  would  make  no  difference  to 
the  general  argument  if  it  were  admitted  that  the  whole  body  consists 
jt  twenty-one  somites  and  the  head  of  seven. 


NOTE  XV.,  CHAPTER  IV.,  p.  199. 
THE  HISTOLOGY  OF  THE  CRAYFISH. 

In  dealing  with  the  histology  of  the  crayfish  I  have  been  obliged  to 
content  myself  with  stating  the  facts  as  they  appear  to  me.  The  discus- 
sion of  the  interpretations  put  upon  these  facts  by  other  observers,  espe- 
cially in  the  case  of  those  tissues,  such  as  muscle,  on  which  there  is  as 
yet  no  complete  agreement  even  as  to  matters  of  observation,  would 
require  a  whole  treatise  to  itself. 


356  NOTES. 

NOTE  XVI.,  CHAPTER  IV.,  p.  221. 
THE  DEVELOPMENT  OF  THE  CRAYFISH. 

The  remark  made  in  the  last  note  applies  still  more  strongly  to  tha 
history  of  the  development  of  the  crayfish.  Notwithstanding  the  mas- 
terly memoir  of  Rathke,  which  constitutes  the  foundation  of  all  our 
knowledge  on  this  subject  ;  the  subsequent  investigations  of  Lereboullet  -t 
and  the  still  more  recent  careful  and  exhaustive  works  of  Reichenbach 
and  Bobretsky,  a  great  many  points  require  further  investigation.  In 
all  its  most  important  features  I  have  reason  to  believe  that  the  account 
rf  the  process  of  development  given  in  the  text,  is  correct. 


NOTE  XVII.,  CHAPTER  VI.,  p.  297. 
PARASITES  OF  CRAYFISHES. 

In  France  and  Germany  crayfishes  (apparently,  however,  only 
A.  nobilis)  are  infested  by  parasites,  belonging  to  the  genus  Bra-ichio- 
bdella.  These  are  minute,  flattened,  vermiform  animals,  somewhat  like 
small  leeches,  from  one-half  to  one-third  of  an  inch  in  length,  which 
attach  themselves  to  the  under  side  of  the  abdomen  (2?.  parasitica),  or 
to  the  gills  (£.  astaci),  and  live  on  the  blood  and  on  the  eggs  of  the 
crayfish.  A  full  account  of  this  parasite,  with  reference  to  the  literature 
of  ihe  subject,  is  given  by  Dormer  ("Ueber  die  Gattung  Branchio 
bdella : "  Zeitschrift  fur  Wiss.  Zoologie,  XV.  1865).  According  to  Gay,  a 
similar  parasite  is  found  on  the  Chilian  crayfish.  I  have  never  met  with 
it  on  the  English  crayfish.  The  Lobster  has  a  somewhat  similar  parasite, 
Histriolddla.  Girard,  in  the  paper  cited  in  the  Bibliography,  gives  a 
curious  account  of  the  manner  in  which  the  little  lamellibranchiate 
mollusk,  Cyclas  fontinalis,  shuts  the  ends  of  the  ambulatory  limbs  of 
crayfishes  which  inhabit  the  same  waters,  between  its  valves,  so  that  l  lie 
crayfish  resembles  a  cat  in  walnut  shells,  and  the  pinched  ends  of  Ihe 
limbs  become  eroded  and  mutilated. 


BIBLIOGRAPHY. 


The  subjoined  list  indicates  the  chief  books  and  memoirs,  in  addition 
to  those  mentioned  in  the  text  and  in  the  Appendix,  which  may  be 
advantageously  consulted  by  any  one  who  wishes  to  study  more  fully 
the  biology  of  the  crayfishes. 

L— NATURAL  HISTORY. 

ROESEL  VON  ROSENBOP.     Der  Monatlich-herausgegeben  Insekten 

Belustigung.     1755. 

CARBONNIEB.    L'Ecrevisse,  Paris,  1869. 
BRANDT  AND  RATZEBURG.     Medizinische  Zoologie.     Bd.  IL,  pp. 

58-70. 

BELL.    British  Stalk-eyed  Crustacea,  1853. 
SOUBEIRAN.     Sur  l'Histoire  naturelle  et  1'Education  des  Ecrevisses. 

Comptes  Rendus,  LX.,  1865. 
CHANTRAN.    Observations  sur  1'Histoire  naturelle  des  Ecrevisses. 

Comptes  Rendus,  LXXL,  1870. 

Sur  la  Fecondation  des  Ecrevisses.    Ibid.,  LXXIV.,  1872. 

— —    Experiences  sur  la  R6g6n6ration  des  Yeux  chez  les  Ecrevisses. 

Ibid.,  LXXVIL,  1873.- 
—  -     Observations  sur  la  Formation  des  Pierres  chez  les  Ecrevisses. 

Ibid.,  LXXVIIL,  1874. 

Sur   le    Mecanisme    de    la    Dissolution   intrastomacale    des 

Concretions  gastriques  des  Ecrevisses.     Ibid.,  LXXVIIL,  1874. 

STEFFENBERG.    Bijdrag  til  kanne  domen  om  flodkraftens  natural 

historia,  1872.    Abstract  in  Zoological  Record,  LX. 
VALLOT.     Sur  1'Ecrevisse  fluviatile  et  sur  son  parasite  I'Astacobdelle 

branchiale.    Comptes  Rendus  Acad.  Sciences.  Dijon.  Me"moires, 

1843-44.     Dijon,  1845. 
PUTNAM.    On  some  of  the  Habits  of  the  Blind  Crayfish.    Proceedings 

Boston  Society  of  Nat.  History,  XVIIL 


358  .  BIBLIOGRAPHY. 

HELLEB.      Ueber  einen    Flusskrebs- albino.    Verhand  d.  Z.  Bot. 

Gesellschaft,  Wien.  Bd.  7, 1857,  and  Bd.  8, 1858. 
LEREBOULLET.     Sur  les  varietes  Rouge  et  Bleue  de  I'Ecrevisse 

fluviatile.     Comptes  Rendus,  XXXIIL,  1857. 
GIRAED.    Quelques  Remarques  sur  1'Astacus  fluviatilis.     Ann.  Soc. 

Entom.  France,  T.  VII.  1859. 

a— ANATOMY  AND  PHYSIOLOGY. 
BRANDT  AND  RATZEBOBG.    Op.  cit. 

MILNE  EDWARDS.     Histoire  naturelle  des  Crustacea.     1834. 
ROLLESTON.     Forms  of  Animal  Life.     1870. 

HUXLEY.     Manual  of  the  Anatomy  of  Vertebrated  Animals.     1877. 
HUXLEY  AND  MARTIN.     Elementary  Biology.     1875. 
SUCKOW.     Anatomisch-Physiologische  Untersuchungen.     1818. 
KROHN.       Verdauungsorgane    des    Krebses.       Gefasssystem    des 

Flusskrebses.     Isis,  1834. 
VON  BAER.     Ueber  die  sogenannte  Erneuerung  des  Magens  der 

Krebse  und  die  Bedeutung  der  Krebssteine.     Mtiller's  Archiv, 

1835. 
OESTERLEN.     Ueber  den  Magen  des  Flusskrebses.    Miiller's  Archiv, 

1840. 
T.   J.    PARKER.       On  the  Stomach  of  the  Freshwater  Crayfish. 

Journal  of  Anatomy  and  Physiology,  1876. 
BARTSCH.      Die  Ernahrangs-  und  Verdauungsorgane  des   Astacus 

leptodactylus.     Budapester  Naturhistor.  Hefte  II.  1878. 
DESZO.      Ueber  das    Herz    des  Flusskrebses  und  des  Hummers. 

Zoologischer  Anzeiger,  L  1878. 
LEREBOULLET.     Note   sur  une   Respiration  anale  observee  chez 

plusieurs  Crustacees.    Mem.  de  la  Societe  d'Histoire  Naturelle  de 

Strasbourg,  IV.,  1850. 
WASSILIEW.     Ueber  die  Niere  des  Flusskrebses.     Zoologischer  An- 

zeiger,  I.  1878. 
LEMOINE.    Recherches  pour  servir  4 1'histoire  des  systemes  nervea*, 

musculaire  et  glandulaire  de  1'Ecrevisse.     Annales  des  Sciences 

Naturelles,  Se.  IV.  T.  15,  1861. 
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fiir  Wiss.  Zoologie,  XXVIL,  1876. 
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Zoologischer  Anzeiger,  I.,  1878. 
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Retina  der  Arthropoden.    1878. 
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1879. 
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Zeitschrift  fiir  Wiss.  Zoologie,  XXX.,  1878. 
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1843. 
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MILNE  EDWARDS.     Op.  cit. 

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F.  New  Zealand. 

MIEES.  Notes  on  the  Genera  Astacoides  and  Paranephrops.  Trans- 
actions of  the  New  Zealand  Institute,  IX.,  1876. 

Paranephrops.  Zoology  of  "  Erebus"  and  "Terror,"  1874.  Cata* 

logue  of  New  Zealand  Crustacea,  1876. 

Annals  of  Natural  History,  1876. 

WOOD-MASON.  On  the  mode  in  which  the  Young  of  the  New  Zealand 
Astacidee  attach  themselves  to  the  Mother.  Ann.  &  Mag. 
Natural  History,  1876 

G-.    Fossil  Astacomorpha. 
OPPEL.     Palaeontologische  Mittheilungen,  1862. 
BELL.    British  Fossil  Crustacea.    Palaeontographical  Society. 
P.  VAN  BENEDEN.    Sur  la  D6couverte  d'un  Homard  fossile  dans 

1'Argile  de  Rupelmonde.  Bulletin  de  1'Acad.  Royale  de  Belgique. 

XXXIIL,  1872. 
VON  DEE  MAECK  und  SCHL^TEE.    Neue  Fische  und  Krebse  von  del 

Kreide  von  Westphalen.     Palseontologica,  XV.  1866. 
COPE.  On  three  extinct  Astaci  from  the  freshwater  tertiary  of  Idaho. 

Proceedings  of  the  American  Philosophical  Society,  XLt  1869-70. 


INDEX. 


A. 

Abdomen,  19,  141 

development  of,  213 
Abdominal  appendages,  143 

development  of,  217 
Abdominal  somite,  characters  of,  1 42 
^Etiology,  47 
AQASSIZ,  308 
Alimentary  canal,  51 

development  of,  213,  222 
Ambulatory  legs,  168 
American  Crayfishes,  243,  247 
Amceba,  285 

Amurland  Crayfishes,  304 
Antenna,  23,  172 

development  of,  214,  218 
Antennule,  23,  173 

development  of,  214,  218 
Anthrapal&mon,  341 
Anus,  29 

Apodeme,  99,  158,  175 
Appendage,  24,  143,  161, 173 

abdominal,  143 

cephalic,  170 

thoracic,  164 
Archenteron,  211 
Arctogaeal  province,  314 
Areola,  235 

ARISTOTLE,  referred  to,  4 
Arteries,  71 


Arteries,  development  of,  224 

Arthrobranchia,  75 

Arthrophragm,  158 

Arthropoda,  279,  284 

Articulations,  95 

Asiatic  Crayfishes,  304 

Astacina,  254 

Agtacoide*,  250,  313 

Astacomorpha,  338 

Attacopsi*,  250,  264 

Astacu*,  division  into  sub-genera, 

290 

Astacii*  angulosu*,  302,  310 
colchicus,  302,  310 
dauricu*,  304,  310 
fluvia  tili*, 

anatomy,  general  account 

of,  17—31 
attachment  of  young    to 

mother,  40,  351 
branchial  formula,  266 
development,  205--226 
distribution,  geographical 

44,  288,  298 

distribution,     chronologi- 
cal, 44 

ecdysis,  32,  350 
general  characters,  6 
growth,  3;,  d49 
habits,  8 


364 


INDEX. 


Artacm  ftimatilis — continued 
histology,  174 
mortality,  127 
muscular  system,  90 
myths  concerning,  44 
name,  origin  of,  13 
nervous  system,  101 
newly  hatched  young,  cha- 
racters of,  219 
nutrition,  48 
occurrence,  5,  8 
organs    of    alimentation, 

51 

circulation,  68 
excretion,  82,  353 
hearing,  116 
reproduction,  128 
respiration,  75,  353 
eight,  118 
smell,  114 
taste,  115 
touch, 113 

prehension  of  food,  49 
putrid,  effect  of  smell  of, 

45 
reproduction  of  lost  limbs, 

38 
reproduction,  sexual,  39, 

128,  135,  350 
sexual  characters,  7,   20, 

32,  145,  241 
•omites  and  appendages, 

143 
systematic  description, 

230 

use  as  food,  10,  289 
varieties,  289 
fontinalis,  290 
japonicus,  304 
klamatJiensis,  305 
leniusculus,  305 


Astacus    leptodactylus,    299,    302 
303,  310,  320 

nigrescens,  244 

nobilis,   290,    295,    296,    299t 
310 

oreganus,  305 

pachypus,  302,  310 

pallipes,  290 

politw,  344 

mxatilis,  290 

Schrenckii,  304,  310 

torrentium,  290,  294,  298,  310 
311 

tristis,  290 

Trombridgii,  305 
Atya,  Atyidce,  331,  336 
Auditory  organ,  116 

setae,  116 
Australian  Crayfishes,  306 

province,  314 

Austrocolumbian  province,  314 
Axius,  271 


BALL,  R.,  quoted,  36 

Basipodite,  143 

BELL,  T.,  quoted,  37,  42 

Bile-duct,  61,  66 

Biological  sciences,  scope  of,  4 

Blastoderm,  207 

Blastomere,  205 

Blastopore,  209 

Blood,  31,  68,  176 

corpuscles,  69,  176 

development  of,  224 

sinuses,  50,  69 

BOBEETSKY,  referred  to,  356 
BOLIVAE,  Dr.,  298 
Branchiae, 

Astacoides,  266 

Astacopsis,  264 


INDEX. 


365 


Branchiae — continued 

Astaau,  25,  75,  265 

development  of,  224 

Cancer,  276 

Honiarus,  257 

Paleemon,  270 

Palinnnu,  264 

Penceus,  267 
Branchial  chamber,  25 

formula, 

Aiiacoides,  266 

Astacopsis,  264 

Aztacw,  266 

Cancer,  277 

hypothetically  complete,  268 

Palcemon,  270 

Palinurits,  265 

Peneeu*,  267 
Branchiobdella,  356 
Branchiostegite,  26 

development  of,  217 
BRAUN,  quoted,  352 
Brazilian  Crayfishes,  306 

C. 

Caecum,  61 

Calcification  of  exoskeleton,  197 

Californian  Crayfishes,  243 

Canibarus,  44,  247,  310,  312 

Cancer,  272,  283 

Carapace,  19 

development  of,  214 
CARBONNIEB,  M.,  quoted,  297,  349, 

350 

Cardia,  52 
Caridina,  330 
Carpopodite,  165 
Cell,  66,  199 
Cell-aggregate,  190.  199 

division,  200 

theory,  202,  204 


Cephalic  appendages,  170 
development  of,  217 

flexure,  163 

somites,  154 
Cephalon,  19, 141 
Cephalothorax,  19 
Cervical  groove,  19 

spines,  234 
CHANTRAN,  M.,  quoted,  348,  850, 

351 

Chelae,  22 

Chilian  Crayfishes,  308 
Chitin,  50 

composition  of,  347 
Chtsrapg,  250 
Chorology,  46 
Circulation,  73 

organs  of,  68 

Common  knowledge  and  science,  3 
Connective  tissue,  178 

development  of,  224 
COPE,  Prof.,  quoted,  316 
Cornea,  118 
Coxopodite,  143 
Coxopoditic  setae,  78 
Crab,  see  Cancer 
Crab's-eye,  see  Gastrolith 
Crangon,  272 
Crayfish,  origin  of  name,  12 

common,  see  AstaeugfluriatUit 
Crayfishes,  Amurland,  304 

Asiatic,  304 

Australian,  306 

Brazilian,  306 

Californian,  243 

Chilian,  308 

definition  of,  254 

Eastern  North  American,  247, 
305 

European,  288.  297 

evolution  of,  331 


3€6 


INDEX. 


Crayfishes,  Figian,  306,  313 

Japanese,  304,  313 

Mascarene,  308,  313 

northern  and  southern,  com- 
pared, 252 

Novozelanian,  306,  313 

southern,  249 

Tasmanian,  306 

Western  North  American,  306, 

313 

Crustacea,  271,  278 
Crystalline  cones,  121 
Cuticle,  33,  60,  175,  192 
Cyclas,  35G 


D. 

Dactylopodite,  165 

Daphiiia,  asexual  reproduction  of 

128 

DAEWIN,  C.,  referred  to,  4 
DE  HAAN,  quoted,  313 
Development,  205 

abdomen,  213 

abdominal  appendages,  217 

alimentary  canal,  213,  222 

antennae,  214,  218 

antennules.  214,  218 

blood  and  blood  vessels,  224 

b^anchiostegite,  217 

carapace,  214 

cephalic  appendages,  217,  219 

connective  tissue,  224 

ear.  225 

eye,  225 

eyestalk,  214,  218 

gills,  224 

heart,  224 

kidney,  224 

labrum,  218 

mandibles,  214 


Development  of  muscles,  224 

nervous  system,  213,  224 

reproductive  organs,  225 

rostrum,  217 

thoracic  appendages,  217,  219 
Digestion,  63 
Distribution,  46 

chronological,  of  crayfishes,  44, 
316,  339 

table  of,  345 

geographical,  of  crayfishes,  44, 
288 

causes  of,  335 

results  of  study  of,  308,  314 
DORMER,  quoted,  356 
DULK,  quoted,  349 


Ear,  116 

development  of,  225 
Ecdysis,  32,  350 
Ecrevisse  a  pieds  blancs,  289,  297 

a  pieds  rouges,  289,  297 
Ectoderm,  141 
Ectostracum,  194 
Edelkrebs,  290 
Endoderm,  141 
Endophragmal  system,  167 
Endopleurite,  158 
Endopodite,  145 
Endoskeleton,  17 
Endosternite,  158 
Endostracum,  194 
Engceus,  250,  306 
Mtwplocytia,  342 
Epiblast,  211 
Epidermis,  140 
Epimeron,1 143 
Epiostracum,  192 
Epipodite,  167 


INDEX. 


867 


Epistoma,  155 
Epithelium.  140,  177 
Eq\iusexceltus,  occurring  with  fossil 

crayfishes,  316 
Eryma,  341 

Evolution  of  crayfishes,  331 
E  icretion,  organs  of,  82 
Exopodite,  145 
Exoskeleton,  17 

chemical  composition,  347 
Eye,  118 

compound,  122 

development  of,  225 
Eye-stalk,  24.  173 

development  of,  214 

F. 

Family,  252 
Fat-cells,  180 
Fibre,  muscular,  185 
Fibril,  muscular,  185 
Figian  Crayfishes,  306 
Filament,  muscular,  185 
Filter  of  stomach,  58 
Flagellum,  167 
Food-yelk,  206 
Foot- jaws,  see  maxillipedes 
Forceps,  22 
Foregut,  61 

development  of,  213,  222 
Fossil  crayfishes.  316 
FOSTER,  Dr.  M.,  referred  to,  110 
France,   consumption  of    crayfish 

in,  10 
Function,  22 

G. 

Gilaxidce,  315 
Gamma  rug,  323 
Ganglion,  103,  105 
Uauglionic  corpuscle,  87.  103 


Gastric  mill,  53 
Gastrolith,  29,  347 

chemical  composition,  349 
Gastrula,  211 
GAY,  quoted,  356 
Genus,  249 
Geographical  distribution,  see  Di* 

tribution 

GERBE,  M.,  quoted,  350 
Germinal  disc,  209 

layer,  206 

spot,  133 

vesicle,  133 

GEESTFELDT,  Dr.,  quoted,  290 
Gills,  see  Branchi® 
GlRARD,  quoted,  356 
GORUP-BESANEZ,  quoted,  353 
Green-gland,  83,  353 

development  of,  224 
GROBBEN,  Dr.,  quoted,  354 
Growth  of  crayfish,  31,  349 
Guanin,  82,  353 
Gullet,  see  (Esophagus 
GUNTHER,  Dr.,  quoted,  315 


H. 

HAGEN,  Dr.,  quoted,  305,  312 
Haplochitonidce,  315 
HARVEY,  quoted.  5 
Head,  see  Cephalon 
Hearing,  organ  of,  116 
Heart,  27,  71 

development  of,  224 
HELLER,  Dr.,  quoted,  298,  330 
Hepatic  duct,  see  Bile  duct 
Hind  gut,  61 

development  of,  214,  223 
Histology,  175 
Ifittrwhdella,  356 
Homaridce,  263 


368 


INDEX. 


Homarm,  13,  42,  257,  332 
Hoinology,    homologous,    homolo- 

gue,  148 
Hoploparia,  342 
Hypoblast,  211 


Jdothea,  323,  334 
Impregnation,  135,  350 
Integument,  50 
Interseptal  zone,  183 
Intestine,  29,  61 
Ischiopodite,  165 


J. 

Japanese  Crayfishes,  313,  314 

Jaws,  23 

JOHNSTON,  J.,  quoted,  42 


K. 

KESSLER,  quoted,  298,  304 
Kidney,  see  Green  gland 
KLUNZINGEE,  referred  to,  330 


Labrum,  51 

development  of,  218 
LAMARCK,  referred  to,  4 
LEREBOULLET,  quoted,  353 
Legs,  ambulatory,  168 
LEMOINE,  referred  to,  353 
LEYDIQ,  referred  to,  115,  353 
Liver,  30,  64 

development  of,  223 

nature  of  secretion.  352 
Lobster,  common,  see  Homariw 

Norway,  see  Neplvrops 


Lobster,  Rock,  see  Palinurv* 
LOVEN,  referred  to,  327 

M. 

Machine,  living,  128 
M'lNTOSH,  Dr.  W.  C.,  quoted,  288 
Mandible,  23,  51,  170 

development  of,  214 
MARTENS,  VON,  306 
Mastodon  mirificm,  occurring  with 

fossil  ciayfishes,  316 
Maxillse,  23,  170 
Maxillipedes,  23,  164 
Medullary  groove,  213 
Megalopa  stage   of   development, 

283 

Meropodite,  165  • 
Mesoblast,  212 
Mesoderm,  141 
Mesophragm,  158 
Metamere,  143 
Metastoma,  61 
Metope,  278 
Midgut,  61 

development  of,  211,  214,  223 
MILNE-EDWARDS,  quoted,  13,  289 
Uollusca,  284 
Morphology,  46,  138 

comparative,  230 
Mortality  of  crayfishes,  128 
Morula,  206 
Mosaic  vision,  122, 354 
Motor  plates,  189 
Mouth,  51 
MULLER,  JOHANNES,  referred  to 

122 
Muscle,  57,  90,  175,  181 

development  of,  224 

histology  of,  90,  181 
Muscles  of  abdomen,  99 


INDEX. 


369 


Muscle*  of  cneiA,  93 

of  stomach,  57 
Myosin,  186 
Myotome,  174 
tfysis,  281,  323 

relicta,    origin    of, 

oculata,  327 
Mysis  stage  oi  development,  280 

N. 
Natural  History,  3 

Philosophy,  3 
Nauplius    stage    of    development, 

215,  280 

Kearctic  province,  314 
Nephrops,  259,  332 
Nerve,  101 

auditory,  117 

optic,  118 
Nerve-cells,  103,  187 

fibres,  101,  188 
Nervous  system,  105 

development  of,  213,  224 

functions  of,  354 

Noble  crayfish,  see  Astacvs  nobilit 
Nomenclature,  binomial,  13,  16 
Norway  lobster,  see  Nephropt 
Novozelanian  province,  314 
Nucleated  cell,  199 
Nacleolus,  187 
Nucleus,  177,  200 

changes  of,  in  cell-division,  200 


0. 

(Esophagus,  51 
Olfactory  organ,  114 
Organ,  22 

Origin  of  crayfish,  evidence  as  to, 
320,  331 

25 


Ovary,  31,  129 

structure  of,  131 
Oviduct,  129 
Oviposition,  351 
Ovisac,  132 
Ovum,  129 

structure  of,  133 


P. 

Palaearctic  province,  314 
Palcemon,  268,  328 
Palinuridce  263 
PalinuniJi,  261,  264 
Palp,  171 

Paranephrops,  250,  306,  313 
Paraphragm,  158 
Parasites  of  crayfish,  356 
Parastacida,  252,  256,  306,  313 
Parastacus,  250,  306 
Pempliix,  341 
Penceu*,  267,  280 
Pericardium,  69 
Perivisceral  cavity,  50 
Phyllobranchia,  271 
Physiology,  46 
Pleurobranchia,  79 
Pleuron,  96,  143 
Podobranchia,  75,  165 
Podophthalmia,  279 
Pore-canals,  195 
Post-orbital  ridge,  233 

spine,  232 

Potamobiida:,  252,  256 
Prawn,  see  Palcemon 
Prehension  of  food,  49 
Procephalic  lobes,  160 

development  of,  213 
Propodite,  165 
Protopodite,  143 
Prototroctet,  315 


INDEX- 


Protozoa, 285 
Psewlastacus,  343 
Pylorus,  52 

B, 

Race,  292 

RATHKE,  quoted,  356 
REAUMUB,  quoted,  33 
Beflex  action,  108 
REICHENBACH,  quoted,  356 
Renal  organ,  see  Green-gland 
Reproduction  of  lost  limbs,  38 
sexual,  39,  128, 135,  350 

Reproductive  organs,  128 
development  of,  225 

Respiration,  anal,  353 

Respiratory  organs,  see  Branchi» 

Retropinna,  315 

ROBIN,  quoted,  352 

Rock  lobster,  see  Palinuru* 

ROESEL   VON    ROSENHOF,    quoted, 

41,43 

RONDOLETIUS,  referred  to,  4 
Rostrum,  157 

development  of,  217 


Salivary  glands,  352 

Salmonidoe,  parallel  between  their 

distribution,  and  that  of  Axta- 

cidas,  315 

Sarcolemma,  90,  182 
SABS,  G.  0.,  referred  to,  327 
SABTOBIUS  VON  WALTEBHAUSEN, 

quoted,  322 

Scaphognathite,  80, 170 
Schizopod  stage  of    development, 

280 

SCULtJTBB,  317 


SCHMIDT,  0.,  quoted,  354 
SCHBANK,  290 
Science,  physical,  3 
Science  and  common  sense,  I 
Segmentation,  174 
Self-causation,  112 
Sensory  organs,  113 
Septal  line,  183 

zone,  183 
Setae,  197 

Shrimp,  see  Crangon 
SIEBOLD,  VON,  referred  to,  331 
Sight,  organ  of,  118 
Sinus,  sternal,  69 
Smell,  organ  of,  114 
Somite,  143, 161,  355 
abdominal,  142 
cephalic,  154 
thoracic,  150 

SOUBEIBAN,  M.,  quoted,  349 
Southern  Crayfishes,  249 
Species,  243,  290 

morphological,  291 
physiological,  296 
Spermatozoa,  129,  135,  354 
Spontaneous  action,  112 
Squame  of  antenna,  172 
Steinkrebs,  see  Astacus  torrent  m 
Sternum,  96,  143 
Stomach,  29,  51 
Stone-crayfish,  see  Astacv*  t&rren- 

tium 

Striated  spindle,  121 
Swimmeret,  20 


T. 

Taste,  organ  of,  115 
Teleology,  47, 137 
Tendon,  92, 175 


INDEX. 


371 


Tergum,  96, 143 
Terminal  plates,  189 
Terminology,  scientific,  14 
Testis,  129 

structure  of,  133 
Thoracic  appendages,  164 

development  of,  217 

somites,  150 
Thorax,  19,  141 
Tissue,  175 
Touch,  organ  of,  113 
Transfonnism,  318 
TBEVTRANUS,  referred  to,  4 
Tribe,  252 

Trichobranchiae,  263 
Troglocai-i*,  337 


V. 

Valves  of  heart,  73 
of  stomach,  59 
VAN  HELMONT,  quoted,  45 
Variety,  290,  292 
Vas  deferens,  130 
Vent,  see  Anus 
284 


Vertebrate,  eye  of,  122, 125 
Visual  pyramid,  121 

rod,  121 

Vitelline  membrane,  133 
ViteUus,  133 
Voluntary  action,  112 
VON  DEE  MAECK,  317 


W. 

WAED,  J.,  referred  to,  354 
WASSILIEW,  quoted,  353 
Whirlpool  of  life,  84 
WILL,  quoted,  353 
WOOD-MASON,  quoted,  44 


Y. 

Yelk,  133 
Yelk-division,  205 
Young  of  Agtacw,  newly  hatched, 
characters  of,  219 


Zoaea  stage  of  development,  380 


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UNIVERSITY  OF  CALIFORNIA  LIBRARY 


